Minor tweaks

## Summary

Another bug found using [property
testing](https://github.com/astral-sh/ruff/pull/14178).

## Test Plan

New unit test

.github/CODEOWNERS

@@ -17,5 +17,5 @@
 /scripts/fuzz-parser/ @AlexWaygood
 
 # red-knot
-/crates/red_knot* @carljm @MichaReiser @AlexWaygood
-/crates/ruff_db/ @carljm @MichaReiser @AlexWaygood
+/crates/red_knot* @carljm @MichaReiser @AlexWaygood @sharkdp
+/crates/ruff_db/ @carljm @MichaReiser @AlexWaygood @sharkdp

.github/workflows/ci.yaml

@@ -16,7 +16,7 @@ env:
   CARGO_TERM_COLOR: always
   RUSTUP_MAX_RETRIES: 10
   PACKAGE_NAME: ruff
-  PYTHON_VERSION: "3.11"
+  PYTHON_VERSION: "3.12"
 
 jobs:
   determine_changes:

.github/workflows/publish-playground.yml

@@ -47,7 +47,7 @@ jobs:
         working-directory: playground
       - name: "Deploy to Cloudflare Pages"
         if: ${{ env.CF_API_TOKEN_EXISTS == 'true' }}
-        uses: cloudflare/wrangler-action@v3.9.0
+        uses: cloudflare/wrangler-action@v3.11.0
         with:
           apiToken: ${{ secrets.CF_API_TOKEN }}
           accountId: ${{ secrets.CF_ACCOUNT_ID }}

.github/workflows/publish-pypi.yml

@@ -21,14 +21,12 @@ jobs:
       # For PyPI's trusted publishing.
       id-token: write
     steps:
+      - name: "Install uv"
+        uses: astral-sh/setup-uv@v3
       - uses: actions/download-artifact@v4
         with:
           pattern: wheels-*
           path: wheels
           merge-multiple: true
       - name: Publish to PyPi
-        uses: pypa/gh-action-pypi-publish@release/v1
-        with:
-          skip-existing: true
-          packages-dir: wheels
-          verbose: true
+        run: uv publish -v wheels/*

.pre-commit-config.yaml

@@ -17,7 +17,7 @@ exclude: |
 
 repos:
   - repo: https://github.com/abravalheri/validate-pyproject
-    rev: v0.21
+    rev: v0.22
     hooks:
       - id: validate-pyproject
 
@@ -51,11 +51,15 @@ repos:
       - id: blacken-docs
         args: ["--pyi", "--line-length", "130"]
         files: '^crates/.*/resources/mdtest/.*\.md'
+        exclude: |
+          (?x)^(
+            .*?invalid(_.+)_syntax.md
+          )$
         additional_dependencies:
           - black==24.10.0
 
   - repo: https://github.com/crate-ci/typos
-    rev: v1.26.0
+    rev: v1.27.0
     hooks:
       - id: typos
 
@@ -69,7 +73,7 @@ repos:
         pass_filenames: false # This makes it a lot faster
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.7.0
+    rev: v0.7.2
     hooks:
       - id: ruff-format
       - id: ruff

CHANGELOG.md

@@ -1,5 +1,68 @@
 # Changelog
 
+## 0.7.3
+
+### Preview features
+
+- Formatter: Disallow single-line implicit concatenated strings ([#13928](https://github.com/astral-sh/ruff/pull/13928))
+- \[`flake8-pyi`\] Include all Python file types for `PYI006` and `PYI066` ([#14059](https://github.com/astral-sh/ruff/pull/14059))
+- \[`flake8-simplify`\] Implement `split-of-static-string` (`SIM905`) ([#14008](https://github.com/astral-sh/ruff/pull/14008))
+- \[`refurb`\] Implement `subclass-builtin` (`FURB189`) ([#14105](https://github.com/astral-sh/ruff/pull/14105))
+- \[`ruff`\] Improve diagnostic messages and docs (`RUF031`, `RUF032`, `RUF034`) ([#14068](https://github.com/astral-sh/ruff/pull/14068))
+
+### Rule changes
+
+- Detect items that hash to same value in duplicate sets (`B033`, `PLC0208`) ([#14064](https://github.com/astral-sh/ruff/pull/14064))
+- \[`eradicate`\] Better detection of IntelliJ language injection comments (`ERA001`) ([#14094](https://github.com/astral-sh/ruff/pull/14094))
+- \[`flake8-pyi`\] Add autofix for `docstring-in-stub` (`PYI021`) ([#14150](https://github.com/astral-sh/ruff/pull/14150))
+- \[`flake8-pyi`\] Update `duplicate-literal-member` (`PYI062`) to alawys provide an autofix ([#14188](https://github.com/astral-sh/ruff/pull/14188))
+- \[`pyflakes`\] Detect items that hash to same value in duplicate dictionaries (`F601`) ([#14065](https://github.com/astral-sh/ruff/pull/14065))
+- \[`ruff`\] Fix false positive for decorators (`RUF028`) ([#14061](https://github.com/astral-sh/ruff/pull/14061))
+
+### Bug fixes
+
+- Avoid parsing joint rule codes as distinct codes in `# noqa` ([#12809](https://github.com/astral-sh/ruff/pull/12809))
+- \[`eradicate`\] ignore `# language=` in commented-out-code rule (ERA001) ([#14069](https://github.com/astral-sh/ruff/pull/14069))
+- \[`flake8-bugbear`\] - do not run `mutable-argument-default` on stubs (`B006`) ([#14058](https://github.com/astral-sh/ruff/pull/14058))
+- \[`flake8-builtins`\] Skip lambda expressions in `builtin-argument-shadowing (A002)` ([#14144](https://github.com/astral-sh/ruff/pull/14144))
+- \[`flake8-comprehension`\] Also remove trailing comma while fixing `C409` and `C419` ([#14097](https://github.com/astral-sh/ruff/pull/14097))
+- \[`flake8-simplify`\] Allow `open` without context manager in `return` statement (`SIM115`) ([#14066](https://github.com/astral-sh/ruff/pull/14066))
+- \[`pylint`\] Respect hash-equivalent literals in `iteration-over-set` (`PLC0208`) ([#14063](https://github.com/astral-sh/ruff/pull/14063))
+- \[`pylint`\] Update known dunder methods for Python 3.13 (`PLW3201`) ([#14146](https://github.com/astral-sh/ruff/pull/14146))
+- \[`pyupgrade`\] - ignore kwarg unpacking for `UP044` ([#14053](https://github.com/astral-sh/ruff/pull/14053))
+- \[`refurb`\] Parse more exotic decimal strings in `verbose-decimal-constructor` (`FURB157`) ([#14098](https://github.com/astral-sh/ruff/pull/14098))
+
+### Documentation
+
+- Add links to missing related options within rule documentations ([#13971](https://github.com/astral-sh/ruff/pull/13971))
+- Add rule short code to mkdocs tags to allow searching via rule codes ([#14040](https://github.com/astral-sh/ruff/pull/14040))
+
+## 0.7.2
+
+### Preview features
+
+- Fix formatting of single with-item with trailing comment ([#14005](https://github.com/astral-sh/ruff/pull/14005))
+- \[`pyupgrade`\] Add PEP 646 `Unpack` conversion to `*` with fix (`UP044`) ([#13988](https://github.com/astral-sh/ruff/pull/13988))
+
+### Rule changes
+
+- Regenerate `known_stdlibs.rs` with stdlibs 2024.10.25 ([#13963](https://github.com/astral-sh/ruff/pull/13963))
+- \[`flake8-no-pep420`\] Skip namespace package enforcement for PEP 723 scripts (`INP001`) ([#13974](https://github.com/astral-sh/ruff/pull/13974))
+
+### Server
+
+- Fix server panic when undoing an edit ([#14010](https://github.com/astral-sh/ruff/pull/14010))
+
+### Bug fixes
+
+- Fix issues in discovering ruff in pip build environments ([#13881](https://github.com/astral-sh/ruff/pull/13881))
+- \[`flake8-type-checking`\] Fix false positive for `singledispatchmethod` (`TCH003`) ([#13941](https://github.com/astral-sh/ruff/pull/13941))
+- \[`flake8-type-checking`\] Treat return type of `singledispatch` as runtime-required (`TCH003`) ([#13957](https://github.com/astral-sh/ruff/pull/13957))
+
+### Documentation
+
+- \[`flake8-simplify`\] Include caveats of enabling `if-else-block-instead-of-if-exp` (`SIM108`) ([#14019](https://github.com/astral-sh/ruff/pull/14019))
+
 ## 0.7.1
 
 ### Preview features

Cargo.lock

@@ -123,9 +123,9 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.90"
+version = "1.0.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "37bf3594c4c988a53154954629820791dde498571819ae4ca50ca811e060cc95"
+checksum = "74f37166d7d48a0284b99dd824694c26119c700b53bf0d1540cdb147dbdaaf13"
 
 [[package]]
 name = "append-only-vec"
@@ -407,7 +407,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -687,7 +687,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "strsim 0.10.0",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -698,7 +698,7 @@ checksum = "a668eda54683121533a393014d8692171709ff57a7d61f187b6e782719f8933f"
 dependencies = [
  "darling_core",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1162,12 +1162,12 @@ dependencies = [
 
 [[package]]
 name = "indexmap"
-version = "2.5.0"
+version = "2.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "68b900aa2f7301e21c36462b170ee99994de34dff39a4a6a528e80e7376d07e5"
+checksum = "707907fe3c25f5424cce2cb7e1cbcafee6bdbe735ca90ef77c29e84591e5b9da"
 dependencies = [
  "equivalent",
- "hashbrown 0.14.5",
+ "hashbrown 0.15.0",
  "serde",
 ]
 
@@ -1193,9 +1193,9 @@ checksum = "b248f5224d1d606005e02c97f5aa4e88eeb230488bcc03bc9ca4d7991399f2b5"
 
 [[package]]
 name = "inotify"
-version = "0.9.6"
+version = "0.10.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f8069d3ec154eb856955c1c0fbffefbf5f3c40a104ec912d4797314c1801abff"
+checksum = "fdd168d97690d0b8c412d6b6c10360277f4d7ee495c5d0d5d5fe0854923255cc"
 dependencies = [
  "bitflags 1.3.2",
  "inotify-sys",
@@ -1213,9 +1213,9 @@ dependencies = [
 
 [[package]]
 name = "insta"
-version = "1.40.0"
+version = "1.41.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6593a41c7a73841868772495db7dc1e8ecab43bb5c0b6da2059246c4b506ab60"
+checksum = "7e9ffc4d4892617c50a928c52b2961cb5174b6fc6ebf252b2fac9d21955c48b8"
 dependencies = [
  "console",
  "globset",
@@ -1267,7 +1267,7 @@ dependencies = [
  "Inflector",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1393,7 +1393,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2ae40017ac09cd2c6a53504cb3c871c7f2b41466eac5bc66ba63f39073b467b"
 dependencies = [
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1532,14 +1532,15 @@ dependencies = [
 
 [[package]]
 name = "mio"
-version = "0.8.11"
+version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a4a650543ca06a924e8b371db273b2756685faae30f8487da1b56505a8f78b0c"
+checksum = "80e04d1dcff3aae0704555fe5fee3bcfaf3d1fdf8a7e521d5b9d2b42acb52cec"
 dependencies = [
+ "hermit-abi",
  "libc",
  "log",
  "wasi",
- "windows-sys 0.48.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1593,12 +1594,11 @@ dependencies = [
 
 [[package]]
 name = "notify"
-version = "6.1.1"
+version = "7.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6205bd8bb1e454ad2e27422015fb5e4f2bcc7e08fa8f27058670d208324a4d2d"
+checksum = "c533b4c39709f9ba5005d8002048266593c1cfaf3c5f0739d5b8ab0c6c504009"
 dependencies = [
  "bitflags 2.6.0",
- "crossbeam-channel",
  "filetime",
  "fsevent-sys",
  "inotify",
@@ -1606,8 +1606,18 @@ dependencies = [
  "libc",
  "log",
  "mio",
+ "notify-types",
  "walkdir",
- "windows-sys 0.48.0",
+ "windows-sys 0.52.0",
+]
+
+[[package]]
+name = "notify-types"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7393c226621f817964ffb3dc5704f9509e107a8b024b489cc2c1b217378785df"
+dependencies = [
+ "instant",
 ]
 
 [[package]]
@@ -1786,9 +1796,9 @@ dependencies = [
 
 [[package]]
 name = "pep440_rs"
-version = "0.7.1"
+version = "0.7.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7c8ee724d21f351f9d47276614ac9710975db827ba9fe2ca5a517ba648193307"
+checksum = "0922a442c78611fa8c5ed6065d2d898a820cf12fa90604217fdb2d01675efec7"
 dependencies = [
  "serde",
  "unicode-width 0.2.0",
@@ -1848,7 +1858,7 @@ dependencies = [
  "pest_meta",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -1963,9 +1973,9 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.88"
+version = "1.0.89"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7c3a7fc5db1e57d5a779a352c8cdb57b29aa4c40cc69c3a68a7fedc815fbf2f9"
+checksum = "f139b0662de085916d1fb67d2b4169d1addddda1919e696f3252b740b629986e"
 dependencies = [
  "unicode-ident",
 ]
@@ -2102,6 +2112,7 @@ dependencies = [
  "countme",
  "dir-test",
  "hashbrown 0.15.0",
+ "indexmap",
  "insta",
  "itertools 0.13.0",
  "memchr",
@@ -2247,9 +2258,9 @@ dependencies = [
 
 [[package]]
 name = "regex"
-version = "1.11.0"
+version = "1.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "38200e5ee88914975b69f657f0801b6f6dccafd44fd9326302a4aaeecfacb1d8"
+checksum = "b544ef1b4eac5dc2db33ea63606ae9ffcfac26c1416a2806ae0bf5f56b201191"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -2306,7 +2317,7 @@ dependencies = [
 
 [[package]]
 name = "ruff"
-version = "0.7.1"
+version = "0.7.3"
 dependencies = [
  "anyhow",
  "argfile",
@@ -2523,7 +2534,7 @@ dependencies = [
 
 [[package]]
 name = "ruff_linter"
-version = "0.7.1"
+version = "0.7.3"
 dependencies = [
  "aho-corasick",
  "annotate-snippets 0.9.2",
@@ -2546,7 +2557,7 @@ dependencies = [
  "natord",
  "path-absolutize",
  "pathdiff",
- "pep440_rs 0.7.1",
+ "pep440_rs 0.7.2",
  "pyproject-toml",
  "quick-junit",
  "regex",
@@ -2590,7 +2601,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "ruff_python_trivia",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -2620,12 +2631,14 @@ dependencies = [
  "compact_str",
  "is-macro",
  "itertools 0.13.0",
+ "memchr",
  "ruff_cache",
  "ruff_macros",
  "ruff_python_trivia",
  "ruff_source_file",
  "ruff_text_size",
  "rustc-hash 2.0.0",
+ "salsa",
  "schemars",
  "serde",
 ]
@@ -2748,7 +2761,6 @@ dependencies = [
  "ruff_python_ast",
  "ruff_python_parser",
  "ruff_python_stdlib",
- "ruff_source_file",
  "ruff_text_size",
  "rustc-hash 2.0.0",
  "schemars",
@@ -2779,7 +2791,6 @@ dependencies = [
  "insta",
  "ruff_python_parser",
  "ruff_python_trivia",
- "ruff_source_file",
  "ruff_text_size",
 ]
 
@@ -2838,7 +2849,7 @@ dependencies = [
 
 [[package]]
 name = "ruff_wasm"
-version = "0.7.1"
+version = "0.7.3"
 dependencies = [
  "console_error_panic_hook",
  "console_log",
@@ -2877,7 +2888,7 @@ dependencies = [
  "matchit",
  "path-absolutize",
  "path-slash",
- "pep440_rs 0.7.1",
+ "pep440_rs 0.7.2",
  "regex",
  "ruff_cache",
  "ruff_formatter",
@@ -3009,7 +3020,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
  "synstructure",
 ]
 
@@ -3043,7 +3054,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "serde_derive_internals",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3066,9 +3077,9 @@ checksum = "1c107b6f4780854c8b126e228ea8869f4d7b71260f962fefb57b996b8959ba6b"
 
 [[package]]
 name = "serde"
-version = "1.0.210"
+version = "1.0.214"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c8e3592472072e6e22e0a54d5904d9febf8508f65fb8552499a1abc7d1078c3a"
+checksum = "f55c3193aca71c12ad7890f1785d2b73e1b9f63a0bbc353c08ef26fe03fc56b5"
 dependencies = [
  "serde_derive",
 ]
@@ -3086,13 +3097,13 @@ dependencies = [
 
 [[package]]
 name = "serde_derive"
-version = "1.0.210"
+version = "1.0.214"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "243902eda00fad750862fc144cea25caca5e20d615af0a81bee94ca738f1df1f"
+checksum = "de523f781f095e28fa605cdce0f8307e451cc0fd14e2eb4cd2e98a355b147766"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3103,7 +3114,7 @@ checksum = "330f01ce65a3a5fe59a60c82f3c9a024b573b8a6e875bd233fe5f934e71d54e3"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3126,7 +3137,7 @@ checksum = "6c64451ba24fc7a6a2d60fc75dd9c83c90903b19028d4eff35e88fc1e86564e9"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3167,7 +3178,7 @@ dependencies = [
  "darling",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3269,7 +3280,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "rustversion",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3291,9 +3302,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.82"
+version = "2.0.87"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "83540f837a8afc019423a8edb95b52a8effe46957ee402287f4292fae35be021"
+checksum = "25aa4ce346d03a6dcd68dd8b4010bcb74e54e62c90c573f394c46eae99aba32d"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3308,7 +3319,7 @@ checksum = "c8af7666ab7b6390ab78131fb5b0fce11d6b7a6951602017c35fa82800708971"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3371,7 +3382,7 @@ dependencies = [
  "cfg-if",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3382,28 +3393,28 @@ checksum = "5c89e72a01ed4c579669add59014b9a524d609c0c88c6a585ce37485879f6ffb"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
  "test-case-core",
 ]
 
 [[package]]
 name = "thiserror"
-version = "1.0.64"
+version = "1.0.67"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d50af8abc119fb8bb6dbabcfa89656f46f84aa0ac7688088608076ad2b459a84"
+checksum = "3b3c6efbfc763e64eb85c11c25320f0737cb7364c4b6336db90aa9ebe27a0bbd"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "1.0.64"
+version = "1.0.67"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "08904e7672f5eb876eaaf87e0ce17857500934f4981c4a0ab2b4aa98baac7fc3"
+checksum = "b607164372e89797d78b8e23a6d67d5d1038c1c65efd52e1389ef8b77caba2a6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3515,7 +3526,7 @@ checksum = "34704c8d6ebcbc939824180af020566b01a7c01f80641264eba0999f6c2b6be7"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3773,7 +3784,7 @@ checksum = "6b91f57fe13a38d0ce9e28a03463d8d3c2468ed03d75375110ec71d93b449a08"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -3859,7 +3870,7 @@ dependencies = [
  "once_cell",
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
  "wasm-bindgen-shared",
 ]
 
@@ -3893,7 +3904,7 @@ checksum = "26c6ab57572f7a24a4985830b120de1594465e5d500f24afe89e16b4e833ef68"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
  "wasm-bindgen-backend",
  "wasm-bindgen-shared",
 ]
@@ -3927,7 +3938,7 @@ checksum = "c97b2ef2c8d627381e51c071c2ab328eac606d3f69dd82bcbca20a9e389d95f0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]
@@ -4215,7 +4226,7 @@ checksum = "9ce1b18ccd8e73a9321186f97e46f9f04b778851177567b1975109d26a08d2a6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.82",
+ "syn 2.0.87",
 ]
 
 [[package]]

Cargo.toml

@@ -81,6 +81,7 @@ hashbrown = { version = "0.15.0", default-features = false, features = [
 ignore = { version = "0.4.22" }
 imara-diff = { version = "0.1.5" }
 imperative = { version = "1.0.4" }
+indexmap = {version = "2.6.0" }
 indicatif = { version = "0.17.8" }
 indoc = { version = "2.0.4" }
 insta = { version = "1.35.1" }
@@ -101,7 +102,7 @@ matchit = { version = "0.8.1" }
 memchr = { version = "2.7.1" }
 mimalloc = { version = "0.1.39" }
 natord = { version = "1.0.9" }
-notify = { version = "6.1.1" }
+notify = { version = "7.0.0" }
 ordermap = { version = "0.5.0" }
 path-absolutize = { version = "3.1.1" }
 path-slash = { version = "0.2.1" }
@@ -188,8 +189,9 @@ missing_panics_doc = "allow"
 module_name_repetitions = "allow"
 must_use_candidate = "allow"
 similar_names = "allow"
+single_match_else = "allow"
 too_many_lines = "allow"
-# To allow `#[allow(clippy::all)]` in `crates/ruff_python_parser/src/python.rs`.
+# Without the hashes we run into a `rustfmt` bug in some snapshot tests, see #13250
 needless_raw_string_hashes = "allow"
 # Disallowed restriction lints
 print_stdout = "warn"
@@ -202,6 +204,10 @@ get_unwrap = "warn"
 rc_buffer = "warn"
 rc_mutex = "warn"
 rest_pat_in_fully_bound_structs = "warn"
+# nursery rules
+redundant_clone = "warn"
+debug_assert_with_mut_call = "warn"
+unused_peekable = "warn"
 
 [profile.release]
 # Note that we set these explicitly, and these values

README.md

@@ -136,8 +136,8 @@ curl -LsSf https://astral.sh/ruff/install.sh | sh
 powershell -c "irm https://astral.sh/ruff/install.ps1 | iex"
 
 # For a specific version.
-curl -LsSf https://astral.sh/ruff/0.7.1/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.7.1/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.7.3/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.7.3/install.ps1 | iex"
 ```
 
 You can also install Ruff via [Homebrew](https://formulae.brew.sh/formula/ruff), [Conda](https://anaconda.org/conda-forge/ruff),
@@ -170,7 +170,7 @@ Ruff can also be used as a [pre-commit](https://pre-commit.com/) hook via [`ruff
 ```yaml
 - repo: https://github.com/astral-sh/ruff-pre-commit
   # Ruff version.
-  rev: v0.7.1
+  rev: v0.7.3
   hooks:
     # Run the linter.
     - id: ruff
@@ -417,6 +417,7 @@ Ruff is used by a number of major open-source projects and companies, including:
 - [Babel](https://github.com/python-babel/babel)
 - Benchling ([Refac](https://github.com/benchling/refac))
 - [Bokeh](https://github.com/bokeh/bokeh)
+- CrowdCent ([NumerBlox](https://github.com/crowdcent/numerblox)) <!-- typos: ignore -->
 - [Cryptography (PyCA)](https://github.com/pyca/cryptography)
 - CERN ([Indico](https://getindico.io/))
 - [DVC](https://github.com/iterative/dvc)

_typos.toml

@@ -12,6 +12,7 @@ pn = "pn"  # `import panel as pn` is a thing
 poit = "poit"
 BA = "BA" # acronym for "Bad Allowed", used in testing.
 jod = "jod" # e.g., `jod-thread`
+Numer = "Numer" # Library name 'NumerBlox' in "Who's Using Ruff?"
 
 [default]
 extend-ignore-re = [

crates/red_knot/src/main.rs

@@ -5,8 +5,6 @@ use anyhow::{anyhow, Context};
 use clap::Parser;
 use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
-use salsa::plumbing::ZalsaDatabase;
-
 use red_knot_python_semantic::SitePackages;
 use red_knot_server::run_server;
 use red_knot_workspace::db::RootDatabase;
@@ -14,7 +12,9 @@ use red_knot_workspace::watch;
 use red_knot_workspace::watch::WorkspaceWatcher;
 use red_knot_workspace::workspace::settings::Configuration;
 use red_knot_workspace::workspace::WorkspaceMetadata;
+use ruff_db::diagnostic::Diagnostic;
 use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
+use salsa::plumbing::ZalsaDatabase;
 use target_version::TargetVersion;
 
 use crate::logging::{setup_tracing, Verbosity};
@@ -144,7 +144,7 @@ pub fn main() -> ExitStatus {
 }
 
 fn run() -> anyhow::Result<ExitStatus> {
-    let args = Args::parse_from(std::env::args().collect::<Vec<_>>());
+    let args = Args::parse_from(std::env::args());
 
     if matches!(args.command, Some(Command::Server)) {
         return run_server().map(|()| ExitStatus::Success);
@@ -318,8 +318,9 @@ impl MainLoop {
                 } => {
                     let has_diagnostics = !result.is_empty();
                     if check_revision == revision {
+                        #[allow(clippy::print_stdout)]
                         for diagnostic in result {
-                            tracing::error!("{}", diagnostic);
+                            println!("{}", diagnostic.display(db));
                         }
                     } else {
                         tracing::debug!(
@@ -378,7 +379,10 @@ impl MainLoopCancellationToken {
 #[derive(Debug)]
 enum MainLoopMessage {
     CheckWorkspace,
-    CheckCompleted { result: Vec<String>, revision: u64 },
+    CheckCompleted {
+        result: Vec<Box<dyn Diagnostic>>,
+        revision: u64,
+    },
     ApplyChanges(Vec<watch::ChangeEvent>),
     Exit,
 }

crates/red_knot_python_semantic/Cargo.toml

@@ -13,7 +13,7 @@ license = { workspace = true }
 [dependencies]
 ruff_db = { workspace = true }
 ruff_index = { workspace = true }
-ruff_python_ast = { workspace = true }
+ruff_python_ast = { workspace = true, features = ["salsa"] }
 ruff_python_stdlib = { workspace = true }
 ruff_source_file = { workspace = true }
 ruff_text_size = { workspace = true }
@@ -24,7 +24,8 @@ bitflags = { workspace = true }
 camino = { workspace = true }
 compact_str = { workspace = true }
 countme = { workspace = true }
-itertools = { workspace = true}
+indexmap = { workspace = true }
+itertools = { workspace = true }
 ordermap = { workspace = true }
 salsa = { workspace = true }
 thiserror = { workspace = true }
@@ -43,10 +44,9 @@ red_knot_test = { workspace = true }
 red_knot_vendored = { workspace = true }
 
 anyhow = { workspace = true }
-dir-test = {workspace = true}
+dir-test = { workspace = true }
 insta = { workspace = true }
 tempfile = { workspace = true }
 
 [lints]
 workspace = true
-

crates/red_knot_python_semantic/resources/mdtest/.mdformat.toml

@@ -0,0 +1 @@
+wrap = 100

crates/red_knot_python_semantic/resources/mdtest/annotations/starred_expressions.md

@@ -0,0 +1,18 @@
+# Starred expression annotations
+
+Type annotations for `*args` can be starred expressions themselves:
+
+```py
+from typing_extensions import TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+
+def append_int(*args: *Ts) -> tuple[*Ts, int]:
+    # TODO: should show some representation of the variadic generic type
+    reveal_type(args)  # revealed: @Todo
+
+    return (*args, 1)
+
+# TODO should be tuple[Literal[True], Literal["a"], int]
+reveal_type(append_int(True, "a"))  # revealed: @Todo
+```

crates/red_knot_python_semantic/resources/mdtest/annotations/string_annotations.md

@@ -0,0 +1,9 @@
+# String annotations
+
+```py
+def f() -> "int":
+    return 1
+
+# TODO: We do not support string annotations, but we should not panic if we encounter them
+reveal_type(f())  # revealed: @Todo
+```

crates/red_knot_python_semantic/resources/mdtest/assignment/annotations.md

@@ -22,3 +22,91 @@ x: int = "foo"  # error: [invalid-assignment] "Object of type `Literal["foo"]` i
 x: int
 x = "foo"  # error: [invalid-assignment] "Object of type `Literal["foo"]` is not assignable to `int`"
 ```
+
+## Tuple annotations are understood
+
+```py path=module.py
+from typing_extensions import Unpack
+
+a: tuple[()] = ()
+b: tuple[int] = (42,)
+c: tuple[str, int] = ("42", 42)
+d: tuple[tuple[str, str], tuple[int, int]] = (("foo", "foo"), (42, 42))
+e: tuple[str, ...] = ()
+# TODO: we should not emit this error
+# error: [call-possibly-unbound-method] "Method `__class_getitem__` of type `Literal[tuple]` is possibly unbound"
+f: tuple[str, *tuple[int, ...], bytes] = ("42", b"42")
+g: tuple[str, Unpack[tuple[int, ...]], bytes] = ("42", b"42")
+h: tuple[list[int], list[int]] = ([], [])
+i: tuple[str | int, str | int] = (42, 42)
+j: tuple[str | int] = (42,)
+```
+
+```py path=script.py
+from module import a, b, c, d, e, f, g, h, i, j
+
+reveal_type(a)  # revealed: tuple[()]
+reveal_type(b)  # revealed: tuple[int]
+reveal_type(c)  # revealed: tuple[str, int]
+reveal_type(d)  # revealed: tuple[tuple[str, str], tuple[int, int]]
+
+# TODO: homogenous tuples, PEP-646 tuples
+reveal_type(e)  # revealed: @Todo
+reveal_type(f)  # revealed: @Todo
+reveal_type(g)  # revealed: @Todo
+
+# TODO: support more kinds of type expressions in annotations
+reveal_type(h)  # revealed: @Todo
+
+reveal_type(i)  # revealed: tuple[str | int, str | int]
+reveal_type(j)  # revealed: tuple[str | int]
+```
+
+## Incorrect tuple assignments are complained about
+
+```py
+# error: [invalid-assignment] "Object of type `tuple[Literal[1], Literal[2]]` is not assignable to `tuple[()]`"
+a: tuple[()] = (1, 2)
+
+# error: [invalid-assignment] "Object of type `tuple[Literal["foo"]]` is not assignable to `tuple[int]`"
+b: tuple[int] = ("foo",)
+
+# error: [invalid-assignment] "Object of type `tuple[list, Literal["foo"]]` is not assignable to `tuple[str | int, str]`"
+c: tuple[str | int, str] = ([], "foo")
+```
+
+## PEP-604 annotations are supported
+
+```py
+def foo() -> str | int | None:
+    return None
+
+reveal_type(foo())  # revealed: str | int | None
+
+def bar() -> str | str | None:
+    return None
+
+reveal_type(bar())  # revealed: str | None
+
+def baz() -> str | str:
+    return "Hello, world!"
+
+reveal_type(baz())  # revealed: str
+```
+
+## Attribute expressions in type annotations are understood
+
+```py
+import builtins
+
+int = "foo"
+a: builtins.int = 42
+
+# error: [invalid-assignment] "Object of type `Literal["bar"]` is not assignable to `int`"
+b: builtins.int = "bar"
+
+c: builtins.tuple[builtins.tuple[builtins.int, builtins.int], builtins.int] = ((42, 42), 42)
+
+# error: [invalid-assignment] "Object of type `Literal["foo"]` is not assignable to `tuple[tuple[int, int], int]`"
+c: builtins.tuple[builtins.tuple[builtins.int, builtins.int], builtins.int] = "foo"
+```

crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md

@@ -0,0 +1,182 @@
+# Augmented assignment
+
+## Basic
+
+```py
+x = 3
+x -= 1
+reveal_type(x)  # revealed: Literal[2]
+
+x = 1.0
+x /= 2
+reveal_type(x)  # revealed: float
+```
+
+## Dunder methods
+
+```py
+class C:
+    def __isub__(self, other: int) -> str:
+        return "Hello, world!"
+
+x = C()
+x -= 1
+reveal_type(x)  # revealed: str
+
+class C:
+    def __iadd__(self, other: str) -> float:
+        return 1.0
+
+x = C()
+x += "Hello"
+reveal_type(x)  # revealed: float
+```
+
+## Unsupported types
+
+```py
+class C:
+    def __isub__(self, other: str) -> int:
+        return 42
+
+x = C()
+x -= 1
+
+# TODO: should error, once operand type check is implemented
+reveal_type(x)  # revealed: int
+```
+
+## Method union
+
+```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+class Foo:
+    if bool_instance():
+        def __iadd__(self, other: int) -> str:
+            return "Hello, world!"
+    else:
+        def __iadd__(self, other: int) -> int:
+            return 42
+
+f = Foo()
+f += 12
+
+reveal_type(f)  # revealed: str | int
+```
+
+## Partially bound `__iadd__`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class Foo:
+    if bool_instance():
+        def __iadd__(self, other: str) -> int:
+            return 42
+
+f = Foo()
+
+# TODO: We should emit an `unsupported-operator` error here, possibly with the information
+# that `Foo.__iadd__` may be unbound as additional context.
+f += "Hello, world!"
+
+reveal_type(f)  # revealed: int | Unknown
+```
+
+## Partially bound with `__add__`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class Foo:
+    def __add__(self, other: str) -> str:
+        return "Hello, world!"
+    if bool_instance():
+        def __iadd__(self, other: str) -> int:
+            return 42
+
+f = Foo()
+f += "Hello, world!"
+
+reveal_type(f)  # revealed: int | str
+```
+
+## Partially bound target union
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class Foo:
+    def __add__(self, other: int) -> str:
+        return "Hello, world!"
+    if bool_instance():
+        def __iadd__(self, other: int) -> int:
+            return 42
+
+if bool_instance():
+    f = Foo()
+else:
+    f = 42.0
+f += 12
+
+reveal_type(f)  # revealed: int | str | float
+```
+
+## Target union
+
+```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+class Foo:
+    def __iadd__(self, other: int) -> str:
+        return "Hello, world!"
+
+if flag:
+    f = Foo()
+else:
+    f = 42.0
+f += 12
+
+reveal_type(f)  # revealed: str | float
+```
+
+## Partially bound target union with `__add__`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+class Foo:
+    def __add__(self, other: int) -> str:
+        return "Hello, world!"
+    if bool_instance():
+        def __iadd__(self, other: int) -> int:
+            return 42
+
+class Bar:
+    def __add__(self, other: int) -> bytes:
+        return b"Hello, world!"
+
+    def __iadd__(self, other: int) -> float:
+        return 42.0
+
+if flag:
+    f = Foo()
+else:
+    f = Bar()
+f += 12
+
+reveal_type(f)  # revealed: int | str | float
+```

crates/red_knot_python_semantic/resources/mdtest/assignment/unbound.md

@@ -3,16 +3,31 @@
 ## Unbound
 
 ```py
-x = foo
+x = foo  # error: [unresolved-reference] "Name `foo` used when not defined"
 foo = 1
-reveal_type(x)  # revealed: Unbound
+
+# No error `unresolved-reference` diagnostic is reported for `x`. This is
+# desirable because we would get a lot of cascading errors even though there
+# is only one root cause (the unbound variable `foo`).
+
+# revealed: Unknown
+reveal_type(x)
 ```
 
+Note: in this particular example, one could argue that the most likely error would be a wrong order
+of the `x`/`foo` definitions, and so it could be desirable to infer `Literal[1]` for the type of
+`x`. On the other hand, there might be a variable `fob` a little higher up in this file, and the
+actual error might have been just a typo. Inferring `Unknown` thus seems like the safest option.
+
 ## Unbound class variable
 
 Name lookups within a class scope fall back to globals, but lookups of class attributes don't.
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1
 
 class C:
@@ -23,3 +38,22 @@ class C:
 reveal_type(C.x)  # revealed: Literal[2]
 reveal_type(C.y)  # revealed: Literal[1]
 ```
+
+## Possibly unbound in class and global scope
+
+```py
+def bool_instance() -> bool:
+    return True
+
+if bool_instance():
+    x = "abc"
+
+class C:
+    if bool_instance():
+        x = 1
+
+    # error: [possibly-unresolved-reference]
+    y = x
+
+reveal_type(C.y)  # revealed: Literal[1] | Literal["abc"]
+```

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -3,6 +3,11 @@
 ## Union of attributes
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     class C:
         x = 1
@@ -13,3 +18,38 @@ else:
 
 reveal_type(C.x)  # revealed: Literal[1, 2]
 ```
+
+## Inherited attributes
+
+```py
+class A:
+    X = "foo"
+
+class B(A): ...
+class C(B): ...
+
+reveal_type(C.X)  # revealed: Literal["foo"]
+```
+
+## Inherited attributes (multiple inheritance)
+
+```py
+class O: ...
+
+class F(O):
+    X = 56
+
+class E(O):
+    X = 42
+
+class D(O): ...
+class C(D, F): ...
+class B(E, D): ...
+class A(B, C): ...
+
+# revealed: tuple[Literal[A], Literal[B], Literal[E], Literal[C], Literal[D], Literal[F], Literal[O], Literal[object]]
+reveal_type(A.__mro__)
+
+# `E` is earlier in the MRO than `F`, so we should use the type of `E.X`
+reveal_type(A.X)  # revealed: Literal[42]
+```

crates/red_knot_python_semantic/resources/mdtest/binary/instances.md

@@ -9,8 +9,8 @@ For references, see:
 
 ## Operations
 
-We support inference for all Python's binary operators:
-`+`, `-`, `*`, `@`, `/`, `//`, `%`, `**`, `<<`, `>>`, `&`, `^`, and `|`.
+We support inference for all Python's binary operators: `+`, `-`, `*`, `@`, `/`, `//`, `%`, `**`,
+`<<`, `>>`, `&`, `^`, and `|`.
 
 ```py
 class A:
@@ -152,9 +152,8 @@ reveal_type(B() - A())  # revealed: int
 
 ## Non-reflected precedence in general
 
-In general, if the left-hand side defines `__add__` and the right-hand side
-defines `__radd__` and the right-hand side is not a subtype of the left-hand
-side, `lhs.__add__` will take precedence:
+In general, if the left-hand side defines `__add__` and the right-hand side defines `__radd__` and
+the right-hand side is not a subtype of the left-hand side, `lhs.__add__` will take precedence:
 
 ```py
 class A:
@@ -181,9 +180,8 @@ reveal_type(C() + C())  # revealed: int
 
 ## Reflected precedence for subtypes (in some cases)
 
-If the right-hand operand is a subtype of the left-hand operand and has a
-different implementation of the reflected method, the reflected method on the
-right-hand operand takes precedence.
+If the right-hand operand is a subtype of the left-hand operand and has a different implementation
+of the reflected method, the reflected method on the right-hand operand takes precedence.
 
 ```py
 class A:
@@ -204,18 +202,13 @@ reveal_type(A() + B())  # revealed: MyString
 # N.B. Still a subtype of `A`, even though `A` does not appear directly in the class's `__bases__`
 class C(B): ...
 
-# TODO: we currently only understand direct subclasses as subtypes of the superclass.
-# We need to iterate through the full MRO rather than just the class's bases;
-# if we do, we'll understand `C` as a subtype of `A`, and correctly understand this as being
-# `MyString` rather than `str`
-reveal_type(A() + C())  # revealed: str
+reveal_type(A() + C())  # revealed: MyString
 ```
 
 ## Reflected precedence 2
 
-If the right-hand operand is a subtype of the left-hand operand, but does not
-override the reflected method, the left-hand operand's non-reflected method
-still takes precedence:
+If the right-hand operand is a subtype of the left-hand operand, but does not override the reflected
+method, the left-hand operand's non-reflected method still takes precedence:
 
 ```py
 class A:
@@ -232,17 +225,15 @@ reveal_type(A() + B())  # revealed: str
 
 ## Only reflected supported
 
-For example, at runtime, `(1).__add__(1.2)` is `NotImplemented`, but
-`(1.2).__radd__(1) == 2.2`, meaning that `1 + 1.2` succeeds at runtime
-(producing `2.2`). The runtime tries the second one only if the first one
-returns `NotImplemented` to signal failure.
+For example, at runtime, `(1).__add__(1.2)` is `NotImplemented`, but `(1.2).__radd__(1) == 2.2`,
+meaning that `1 + 1.2` succeeds at runtime (producing `2.2`). The runtime tries the second one only
+if the first one returns `NotImplemented` to signal failure.
 
-Typeshed and other stubs annotate dunder-method calls that would return
-`NotImplemented` as being "illegal" calls. `int.__add__` is annotated as only
-"accepting" `int`s, even though it strictly-speaking "accepts" any other object
-without raising an exception -- it will simply return `NotImplemented`,
-allowing the runtime to try the `__radd__` method of the right-hand operand
-as well.
+Typeshed and other stubs annotate dunder-method calls that would return `NotImplemented` as being
+"illegal" calls. `int.__add__` is annotated as only "accepting" `int`s, even though it
+strictly-speaking "accepts" any other object without raising an exception -- it will simply return
+`NotImplemented`, allowing the runtime to try the `__radd__` method of the right-hand operand as
+well.
 
 ```py
 class A:
@@ -308,8 +299,8 @@ reveal_type(y + 4.12)  # revealed: int
 
 ## With literal types
 
-When we have a literal type for one operand, we're able to fall back to the
-instance handling for its instance super-type.
+When we have a literal type for one operand, we're able to fall back to the instance handling for
+its instance super-type.
 
 ```py
 class A:
@@ -348,15 +339,13 @@ reveal_type(literal_string_instance + A())  # revealed: @Todo
 
 ## Operations involving instances of classes inheriting from `Any`
 
-`Any` and `Unknown` represent a set of possible runtime objects, wherein the
-bounds of the set are unknown. Whether the left-hand operand's dunder or the
-right-hand operand's reflected dunder depends on whether the right-hand operand
-is an instance of a class that is a subclass of the left-hand operand's class
-and overrides the reflected dunder. In the following example, because of the
-unknowable nature of `Any`/`Unknown`, we must consider both possibilities:
-`Any`/`Unknown` might resolve to an unknown third class that inherits from `X`
-and overrides `__radd__`; but it also might not. Thus, the correct answer here
-for the `reveal_type` is `int | Unknown`.
+`Any` and `Unknown` represent a set of possible runtime objects, wherein the bounds of the set are
+unknown. Whether the left-hand operand's dunder or the right-hand operand's reflected dunder depends
+on whether the right-hand operand is an instance of a class that is a subclass of the left-hand
+operand's class and overrides the reflected dunder. In the following example, because of the
+unknowable nature of `Any`/`Unknown`, we must consider both possibilities: `Any`/`Unknown` might
+resolve to an unknown third class that inherits from `X` and overrides `__radd__`; but it also might
+not. Thus, the correct answer here for the `reveal_type` is `int | Unknown`.
 
 ```py
 from does_not_exist import Foo  # error: [unresolved-import]
@@ -426,10 +415,9 @@ reveal_type(B() + C())
 
 ### Reflected dunder is not tried between two objects of the same type
 
-For the specific case where the left-hand operand is the exact same type as the
-right-hand operand, the reflected dunder of the right-hand operand is not
-tried; the runtime short-circuits after trying the unreflected dunder of the
-left-hand operand. For context, see
+For the specific case where the left-hand operand is the exact same type as the right-hand operand,
+the reflected dunder of the right-hand operand is not tried; the runtime short-circuits after trying
+the unreflected dunder of the left-hand operand. For context, see
 [this mailing list discussion](https://mail.python.org/archives/list/python-dev@python.org/thread/7NZUCODEAPQFMRFXYRMGJXDSIS3WJYIV/).
 
 ```py

crates/red_knot_python_semantic/resources/mdtest/boolean/short_circuit.md

@@ -0,0 +1,78 @@
+# Short-Circuit Evaluation
+
+## Not all boolean expressions must be evaluated
+
+In `or` expressions, if the left-hand side is truthy, the right-hand side is not evaluated.
+Similarly, in `and` expressions, if the left-hand side is falsy, the right-hand side is not
+evaluated.
+
+```py
+def bool_instance() -> bool:
+    return True
+
+if bool_instance() or (x := 1):
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)  # revealed: Literal[1]
+
+if bool_instance() and (x := 1):
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)  # revealed: Literal[1]
+```
+
+## First expression is always evaluated
+
+```py
+def bool_instance() -> bool:
+    return True
+
+if (x := 1) or bool_instance():
+    reveal_type(x)  # revealed: Literal[1]
+
+if (x := 1) and bool_instance():
+    reveal_type(x)  # revealed: Literal[1]
+```
+
+## Statically known truthiness
+
+```py
+if True or (x := 1):
+    # TODO: infer that the second arm is never executed, and raise `unresolved-reference`.
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)  # revealed: Literal[1]
+
+if True and (x := 1):
+    # TODO: infer that the second arm is always executed, do not raise a diagnostic
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)  # revealed: Literal[1]
+```
+
+## Later expressions can always use variables from earlier expressions
+
+```py
+def bool_instance() -> bool:
+    return True
+
+bool_instance() or (x := 1) or reveal_type(x)  # revealed: Literal[1]
+
+# error: [unresolved-reference]
+bool_instance() or reveal_type(y) or (y := 1)  # revealed: Unknown
+```
+
+## Nested expressions
+
+```py
+def bool_instance() -> bool:
+    return True
+
+if bool_instance() or ((x := 1) and bool_instance()):
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)  # revealed: Literal[1]
+
+if ((y := 1) and bool_instance()) or bool_instance():
+    reveal_type(y)  # revealed: Literal[1]
+
+# error: [possibly-unresolved-reference]
+if (bool_instance() and (z := 1)) or reveal_type(z):  # revealed: Literal[1]
+    # error: [possibly-unresolved-reference]
+    reveal_type(z)  # revealed: Literal[1]
+```

crates/red_knot_python_semantic/resources/mdtest/call/callable_instance.md

@@ -18,3 +18,58 @@ class Unit: ...
 b = Unit()(3.0)  # error: "Object of type `Unit` is not callable"
 reveal_type(b)  # revealed: Unknown
 ```
+
+## Possibly unbound `__call__` method
+
+```py
+def flag() -> bool: ...
+
+class PossiblyNotCallable:
+    if flag():
+        def __call__(self) -> int: ...
+
+a = PossiblyNotCallable()
+result = a()  # error: "Object of type `PossiblyNotCallable` is not callable (possibly unbound `__call__` method)"
+reveal_type(result)  # revealed: int
+```
+
+## Possibly unbound callable
+
+```py
+def flag() -> bool: ...
+
+if flag():
+    class PossiblyUnbound:
+        def __call__(self) -> int: ...
+
+# error: [possibly-unresolved-reference]
+a = PossiblyUnbound()
+reveal_type(a())  # revealed: int
+```
+
+## Non-callable `__call__`
+
+```py
+class NonCallable:
+    __call__ = 1
+
+a = NonCallable()
+# error: "Object of type `NonCallable` is not callable"
+reveal_type(a())  # revealed: Unknown
+```
+
+## Possibly non-callable `__call__`
+
+```py
+def flag() -> bool: ...
+
+class NonCallable:
+    if flag():
+        __call__ = 1
+    else:
+        def __call__(self) -> int: ...
+
+a = NonCallable()
+# error: "Object of type `Literal[1] | Literal[__call__]` is not callable (due to union element `Literal[1]`)"
+reveal_type(a())  # revealed: Unknown | int
+```

crates/red_knot_python_semantic/resources/mdtest/call/function.md

@@ -44,3 +44,16 @@ reveal_type(bar())  # revealed: @Todo
 nonsense = 123
 x = nonsense()  # error: "Object of type `Literal[123]` is not callable"
 ```
+
+## Potentially unbound function
+
+```py
+def flag() -> bool: ...
+
+if flag():
+    def foo() -> int:
+        return 42
+
+# error: [possibly-unresolved-reference]
+reveal_type(foo())  # revealed: int
+```

crates/red_knot_python_semantic/resources/mdtest/call/union.md

@@ -3,6 +3,11 @@
 ## Union of return types
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
 
     def f() -> int:
@@ -21,6 +26,11 @@ reveal_type(f())  # revealed: int | str
 ```py
 from nonexistent import f  # error: [unresolved-import] "Cannot resolve import `nonexistent`"
 
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
 
     def f() -> int:
@@ -34,6 +44,11 @@ reveal_type(f())  # revealed: Unknown | int
 Calling a union with a non-callable element should emit a diagnostic.
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     f = 1
 else:
@@ -50,6 +65,11 @@ reveal_type(x)  # revealed: Unknown | int
 Calling a union with multiple non-callable elements should mention all of them in the diagnostic.
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
+
 if flag:
     f = 1
 elif flag2:
@@ -69,6 +89,11 @@ reveal_type(f())
 Calling a union with no callable elements can emit a simpler diagnostic.
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     f = 1
 else:

crates/red_knot_python_semantic/resources/mdtest/comparison/byte_literals.md

@@ -1,7 +1,7 @@
 # Comparison: Byte literals
 
-These tests assert that we infer precise `Literal` types for comparisons between objects
-inferred as having `Literal` bytes types:
+These tests assert that we infer precise `Literal` types for comparisons between objects inferred as
+having `Literal` bytes types:
 
 ```py
 reveal_type(b"abc" == b"abc")  # revealed: Literal[True]

crates/red_knot_python_semantic/resources/mdtest/comparison/identity_tests.md

@@ -0,0 +1,40 @@
+# Identity tests
+
+```py
+class A: ...
+
+def get_a() -> A: ...
+def get_object() -> object: ...
+
+a1 = get_a()
+a2 = get_a()
+
+n1 = None
+n2 = None
+
+o = get_object()
+
+reveal_type(a1 is a1)  # revealed: bool
+reveal_type(a1 is a2)  # revealed: bool
+
+reveal_type(n1 is n1)  # revealed: Literal[True]
+reveal_type(n1 is n2)  # revealed: Literal[True]
+
+reveal_type(a1 is n1)  # revealed: Literal[False]
+reveal_type(n1 is a1)  # revealed: Literal[False]
+
+reveal_type(a1 is o)  # revealed: bool
+reveal_type(n1 is o)  # revealed: bool
+
+reveal_type(a1 is not a1)  # revealed: bool
+reveal_type(a1 is not a2)  # revealed: bool
+
+reveal_type(n1 is not n1)  # revealed: Literal[False]
+reveal_type(n1 is not n2)  # revealed: Literal[False]
+
+reveal_type(a1 is not n1)  # revealed: Literal[True]
+reveal_type(n1 is not a1)  # revealed: Literal[True]
+
+reveal_type(a1 is not o)  # revealed: bool
+reveal_type(n1 is not o)  # revealed: bool
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/instances/membership_test.md

@@ -0,0 +1,160 @@
+# Comparison: Membership Test
+
+In Python, the term "membership test operators" refers to the operators `in` and `not in`. To
+customize their behavior, classes can implement one of the special methods `__contains__`,
+`__iter__`, or `__getitem__`.
+
+For references, see:
+
+- <https://docs.python.org/3/reference/expressions.html#membership-test-details>
+- <https://docs.python.org/3/reference/datamodel.html#object.__contains__>
+- <https://snarky.ca/unravelling-membership-testing/>
+
+## Implements `__contains__`
+
+Classes can support membership tests by implementing the `__contains__` method:
+
+```py
+class A:
+    def __contains__(self, item: str) -> bool:
+        return True
+
+reveal_type("hello" in A())  # revealed: bool
+reveal_type("hello" not in A())  # revealed: bool
+# TODO: should emit diagnostic, need to check arg type, will fail
+reveal_type(42 in A())  # revealed: bool
+reveal_type(42 not in A())  # revealed: bool
+```
+
+## Implements `__iter__`
+
+Classes that don't implement `__contains__`, but do implement `__iter__`, also support containment
+checks; the needle will be sought in their iterated items:
+
+```py
+class StringIterator:
+    def __next__(self) -> str:
+        return "foo"
+
+class A:
+    def __iter__(self) -> StringIterator:
+        return StringIterator()
+
+reveal_type("hello" in A())  # revealed: bool
+reveal_type("hello" not in A())  # revealed: bool
+reveal_type(42 in A())  # revealed: bool
+reveal_type(42 not in A())  # revealed: bool
+```
+
+## Implements `__getitems__`
+
+The final fallback is to implement `__getitem__` for integer keys. Python will call `__getitem__`
+with `0`, `1`, `2`... until either the needle is found (leading the membership test to evaluate to
+`True`) or `__getitem__` raises `IndexError` (the raised exception is swallowed, but results in the
+membership test evaluating to `False`).
+
+```py
+class A:
+    def __getitem__(self, key: int) -> str:
+        return "foo"
+
+reveal_type("hello" in A())  # revealed: bool
+reveal_type("hello" not in A())  # revealed: bool
+reveal_type(42 in A())  # revealed: bool
+reveal_type(42 not in A())  # revealed: bool
+```
+
+## Wrong Return Type
+
+Python coerces the results of containment checks to `bool`, even if `__contains__` returns a
+non-bool:
+
+```py
+class A:
+    def __contains__(self, item: str) -> str:
+        return "foo"
+
+reveal_type("hello" in A())  # revealed: bool
+reveal_type("hello" not in A())  # revealed: bool
+```
+
+## Literal Result for `in` and `not in`
+
+`__contains__` with a literal return type may result in a `BooleanLiteral` outcome.
+
+```py
+from typing import Literal
+
+class AlwaysTrue:
+    def __contains__(self, item: int) -> Literal[1]:
+        return 1
+
+class AlwaysFalse:
+    def __contains__(self, item: int) -> Literal[""]:
+        return ""
+
+reveal_type(42 in AlwaysTrue())  # revealed: Literal[True]
+reveal_type(42 not in AlwaysTrue())  # revealed: Literal[False]
+
+reveal_type(42 in AlwaysFalse())  # revealed: Literal[False]
+reveal_type(42 not in AlwaysFalse())  # revealed: Literal[True]
+```
+
+## No Fallback for `__contains__`
+
+If `__contains__` is implemented, checking membership of a type it doesn't accept is an error; it
+doesn't result in a fallback to `__iter__` or `__getitem__`:
+
+```py
+class CheckContains: ...
+class CheckIter: ...
+class CheckGetItem: ...
+
+class CheckIterIterator:
+    def __next__(self) -> CheckIter:
+        return CheckIter()
+
+class A:
+    def __contains__(self, item: CheckContains) -> bool:
+        return True
+
+    def __iter__(self) -> CheckIterIterator:
+        return CheckIterIterator()
+
+    def __getitem__(self, key: int) -> CheckGetItem:
+        return CheckGetItem()
+
+reveal_type(CheckContains() in A())  # revealed: bool
+
+# TODO: should emit diagnostic, need to check arg type,
+# should not fall back to __iter__ or __getitem__
+reveal_type(CheckIter() in A())  # revealed: bool
+reveal_type(CheckGetItem() in A())  # revealed: bool
+
+class B:
+    def __iter__(self) -> CheckIterIterator:
+        return CheckIterIterator()
+
+    def __getitem__(self, key: int) -> CheckGetItem:
+        return CheckGetItem()
+
+reveal_type(CheckIter() in B())  # revealed: bool
+# Always use `__iter__`, regardless of iterated type; there's no NotImplemented
+# in this case, so there's no fallback to `__getitem__`
+reveal_type(CheckGetItem() in B())  # revealed: bool
+```
+
+## Invalid Old-Style Iteration
+
+If `__getitem__` is implemented but does not accept integer arguments, then the membership test is
+not supported and should trigger a diagnostic.
+
+```py
+class A:
+    def __getitem__(self, key: str) -> str:
+        return "foo"
+
+# TODO should emit a diagnostic
+reveal_type(42 in A())  # revealed: bool
+reveal_type("hello" in A())  # revealed: bool
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/instances/rich_comparison.md

@@ -0,0 +1,328 @@
+# Comparison: Rich Comparison
+
+Rich comparison operations (`==`, `!=`, `<`, `<=`, `>`, `>=`) in Python are implemented through
+double-underscore methods that allow customization of comparison behavior.
+
+For references, see:
+
+- <https://docs.python.org/3/reference/datamodel.html#object.__lt__>
+- <https://snarky.ca/unravelling-rich-comparison-operators/>
+
+## Rich Comparison Dunder Implementations For Same Class
+
+Classes can support rich comparison by implementing dunder methods like `__eq__`, `__ne__`, etc. The
+most common case involves implementing these methods for the same type:
+
+```py
+from __future__ import annotations
+
+class A:
+    def __eq__(self, other: A) -> int:
+        return 42
+
+    def __ne__(self, other: A) -> float:
+        return 42.0
+
+    def __lt__(self, other: A) -> str:
+        return "42"
+
+    def __le__(self, other: A) -> bytes:
+        return b"42"
+
+    def __gt__(self, other: A) -> list:
+        return [42]
+
+    def __ge__(self, other: A) -> set:
+        return {42}
+
+reveal_type(A() == A())  # revealed: int
+reveal_type(A() != A())  # revealed: float
+reveal_type(A() < A())  # revealed: str
+reveal_type(A() <= A())  # revealed: bytes
+reveal_type(A() > A())  # revealed: list
+reveal_type(A() >= A())  # revealed: set
+```
+
+## Rich Comparison Dunder Implementations for Other Class
+
+In some cases, classes may implement rich comparison dunder methods for comparisons with a different
+type:
+
+```py
+from __future__ import annotations
+
+class A:
+    def __eq__(self, other: B) -> int:
+        return 42
+
+    def __ne__(self, other: B) -> float:
+        return 42.0
+
+    def __lt__(self, other: B) -> str:
+        return "42"
+
+    def __le__(self, other: B) -> bytes:
+        return b"42"
+
+    def __gt__(self, other: B) -> list:
+        return [42]
+
+    def __ge__(self, other: B) -> set:
+        return {42}
+
+class B: ...
+
+reveal_type(A() == B())  # revealed: int
+reveal_type(A() != B())  # revealed: float
+reveal_type(A() < B())  # revealed: str
+reveal_type(A() <= B())  # revealed: bytes
+reveal_type(A() > B())  # revealed: list
+reveal_type(A() >= B())  # revealed: set
+```
+
+## Reflected Comparisons
+
+Fallback to the right-hand side’s comparison methods occurs when the left-hand side does not define
+them. Note: class `B` has its own `__eq__` and `__ne__` methods to override those of `object`, but
+these methods will be ignored here because they require a mismatched operand type.
+
+```py
+from __future__ import annotations
+
+class A:
+    def __eq__(self, other: B) -> int:
+        return 42
+
+    def __ne__(self, other: B) -> float:
+        return 42.0
+
+    def __lt__(self, other: B) -> str:
+        return "42"
+
+    def __le__(self, other: B) -> bytes:
+        return b"42"
+
+    def __gt__(self, other: B) -> list:
+        return [42]
+
+    def __ge__(self, other: B) -> set:
+        return {42}
+
+class B:
+    # To override builtins.object.__eq__ and builtins.object.__ne__
+    # TODO these should emit an invalid override diagnostic
+    def __eq__(self, other: str) -> B:
+        return B()
+
+    def __ne__(self, other: str) -> B:
+        return B()
+
+# TODO: should be `int` and `float`.
+# Need to check arg type and fall back to `rhs.__eq__` and `rhs.__ne__`.
+#
+# Because `object.__eq__` and `object.__ne__` accept `object` in typeshed,
+# this can only happen with an invalid override of these methods,
+# but we still support it.
+reveal_type(B() == A())  # revealed: B
+reveal_type(B() != A())  # revealed: B
+
+reveal_type(B() < A())  # revealed: list
+reveal_type(B() <= A())  # revealed: set
+
+reveal_type(B() > A())  # revealed: str
+reveal_type(B() >= A())  # revealed: bytes
+
+class C:
+    def __gt__(self, other: C) -> int:
+        return 42
+
+    def __ge__(self, other: C) -> float:
+        return 42.0
+
+reveal_type(C() < C())  # revealed: int
+reveal_type(C() <= C())  # revealed: float
+```
+
+## Reflected Comparisons with Subclasses
+
+When subclasses override comparison methods, these overridden methods take precedence over those in
+the parent class. Class `B` inherits from `A` and redefines comparison methods to return types other
+than `A`.
+
+```py
+from __future__ import annotations
+
+class A:
+    def __eq__(self, other: A) -> A:
+        return A()
+
+    def __ne__(self, other: A) -> A:
+        return A()
+
+    def __lt__(self, other: A) -> A:
+        return A()
+
+    def __le__(self, other: A) -> A:
+        return A()
+
+    def __gt__(self, other: A) -> A:
+        return A()
+
+    def __ge__(self, other: A) -> A:
+        return A()
+
+class B(A):
+    def __eq__(self, other: A) -> int:
+        return 42
+
+    def __ne__(self, other: A) -> float:
+        return 42.0
+
+    def __lt__(self, other: A) -> str:
+        return "42"
+
+    def __le__(self, other: A) -> bytes:
+        return b"42"
+
+    def __gt__(self, other: A) -> list:
+        return [42]
+
+    def __ge__(self, other: A) -> set:
+        return {42}
+
+reveal_type(A() == B())  # revealed: int
+reveal_type(A() != B())  # revealed: float
+
+reveal_type(A() < B())  # revealed: list
+reveal_type(A() <= B())  # revealed: set
+
+reveal_type(A() > B())  # revealed: str
+reveal_type(A() >= B())  # revealed: bytes
+```
+
+## Reflected Comparisons with Subclass But Falls Back to LHS
+
+In the case of a subclass, the right-hand side has priority. However, if the overridden dunder
+method has an mismatched type to operand, the comparison will fall back to the left-hand side.
+
+```py
+from __future__ import annotations
+
+class A:
+    def __lt__(self, other: A) -> A:
+        return A()
+
+    def __gt__(self, other: A) -> A:
+        return A()
+
+class B(A):
+    def __lt__(self, other: int) -> B:
+        return B()
+
+    def __gt__(self, other: int) -> B:
+        return B()
+
+# TODO: should be `A`, need to check argument type and fall back to LHS method
+reveal_type(A() < B())  # revealed: B
+reveal_type(A() > B())  # revealed: B
+```
+
+## Operations involving instances of classes inheriting from `Any`
+
+`Any` and `Unknown` represent a set of possible runtime objects, wherein the bounds of the set are
+unknown. Whether the left-hand operand's dunder or the right-hand operand's reflected dunder depends
+on whether the right-hand operand is an instance of a class that is a subclass of the left-hand
+operand's class and overrides the reflected dunder. In the following example, because of the
+unknowable nature of `Any`/`Unknown`, we must consider both possibilities: `Any`/`Unknown` might
+resolve to an unknown third class that inherits from `X` and overrides `__gt__`; but it also might
+not. Thus, the correct answer here for the `reveal_type` is `int | Unknown`.
+
+(This test is referenced from `mdtest/binary/instances.md`)
+
+```py
+from does_not_exist import Foo  # error: [unresolved-import]
+
+reveal_type(Foo)  # revealed: Unknown
+
+class X:
+    def __lt__(self, other: object) -> int:
+        return 42
+
+class Y(Foo): ...
+
+# TODO: Should be `int | Unknown`; see above discussion.
+reveal_type(X() < Y())  # revealed: int
+```
+
+## Equality and Inequality Fallback
+
+This test confirms that `==` and `!=` comparisons default to identity comparisons (`is`, `is not`)
+when argument types do not match the method signature.
+
+Please refer to the [docs](https://docs.python.org/3/reference/datamodel.html#object.__eq__)
+
+```py
+from __future__ import annotations
+
+class A:
+    # TODO both these overrides should emit invalid-override diagnostic
+    def __eq__(self, other: int) -> A:
+        return A()
+
+    def __ne__(self, other: int) -> A:
+        return A()
+
+# TODO: it should be `bool`, need to check arg type and fall back to `is` and `is not`
+reveal_type(A() == A())  # revealed: A
+reveal_type(A() != A())  # revealed: A
+```
+
+## Object Comparisons with Typeshed
+
+```py
+class A: ...
+
+reveal_type(A() == object())  # revealed: bool
+reveal_type(A() != object())  # revealed: bool
+reveal_type(object() == A())  # revealed: bool
+reveal_type(object() != A())  # revealed: bool
+
+# error: [unsupported-operator] "Operator `<` is not supported for types `A` and `object`"
+# revealed: Unknown
+reveal_type(A() < object())
+```
+
+## Numbers Comparison with typeshed
+
+```py
+reveal_type(1 == 1.0)  # revealed: bool
+reveal_type(1 != 1.0)  # revealed: bool
+reveal_type(1 < 1.0)  # revealed: bool
+reveal_type(1 <= 1.0)  # revealed: bool
+reveal_type(1 > 1.0)  # revealed: bool
+reveal_type(1 >= 1.0)  # revealed: bool
+
+reveal_type(1 == 2j)  # revealed: bool
+reveal_type(1 != 2j)  # revealed: bool
+
+# TODO: should be Unknown and emit diagnostic,
+# need to check arg type and should be failed
+reveal_type(1 < 2j)  # revealed: bool
+reveal_type(1 <= 2j)  # revealed: bool
+reveal_type(1 > 2j)  # revealed: bool
+reveal_type(1 >= 2j)  # revealed: bool
+
+def bool_instance() -> bool:
+    return True
+
+def int_instance() -> int:
+    return 42
+
+x = bool_instance()
+y = int_instance()
+
+reveal_type(x < y)  # revealed: bool
+reveal_type(y < x)  # revealed: bool
+reveal_type(4.2 < x)  # revealed: bool
+reveal_type(x < 4.2)  # revealed: bool
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/integers.md

@@ -12,16 +12,18 @@ reveal_type(1 is 1)  # revealed: bool
 reveal_type(1 is not 1)  # revealed: bool
 reveal_type(1 is 2)  # revealed: Literal[False]
 reveal_type(1 is not 7)  # revealed: Literal[True]
-reveal_type(1 <= "" and 0 < 1)  # revealed: @Todo | Literal[True]
+# TODO: should be Unknown, and emit diagnostic, once we check call argument types
+reveal_type(1 <= "" and 0 < 1)  # revealed: bool
 ```
 
 ## Integer instance
 
 ```py
 # TODO: implement lookup of `__eq__` on typeshed `int` stub.
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
-reveal_type(1 == int_instance())  # revealed: @Todo
+reveal_type(1 == int_instance())  # revealed: bool
 reveal_type(9 < int_instance())  # revealed: bool
 reveal_type(int_instance() < int_instance())  # revealed: bool
 ```

crates/red_knot_python_semantic/resources/mdtest/comparison/intersections.md

@@ -0,0 +1,155 @@
+# Comparison: Intersections
+
+## Positive contributions
+
+If we have an intersection type `A & B` and we get a definitive true/false answer for one of the
+types, we can infer that the result for the intersection type is also true/false:
+
+```py
+class Base: ...
+
+class Child1(Base):
+    def __eq__(self, other) -> Literal[True]:
+        return True
+
+class Child2(Base): ...
+
+def get_base() -> Base: ...
+
+x = get_base()
+c1 = Child1()
+
+# Create an intersection type through narrowing:
+if isinstance(x, Child1):
+    if isinstance(x, Child2):
+        reveal_type(x)  # revealed: Child1 & Child2
+
+        reveal_type(x == 1)  # revealed: Literal[True]
+
+        # Other comparison operators fall back to the base type:
+        reveal_type(x > 1)  # revealed: bool
+        reveal_type(x is c1)  # revealed: bool
+```
+
+## Negative contributions
+
+Negative contributions to the intersection type only allow simplifications in a few special cases
+(equality and identity comparisons).
+
+### Equality comparisons
+
+#### Literal strings
+
+```py
+x = "x" * 1_000_000_000
+y = "y" * 1_000_000_000
+reveal_type(x)  # revealed: LiteralString
+
+if x != "abc":
+    reveal_type(x)  # revealed: LiteralString & ~Literal["abc"]
+
+    reveal_type(x == "abc")  # revealed: Literal[False]
+    reveal_type("abc" == x)  # revealed: Literal[False]
+    reveal_type(x == "something else")  # revealed: bool
+    reveal_type("something else" == x)  # revealed: bool
+
+    reveal_type(x != "abc")  # revealed: Literal[True]
+    reveal_type("abc" != x)  # revealed: Literal[True]
+    reveal_type(x != "something else")  # revealed: bool
+    reveal_type("something else" != x)  # revealed: bool
+
+    reveal_type(x == y)  # revealed: bool
+    reveal_type(y == x)  # revealed: bool
+    reveal_type(x != y)  # revealed: bool
+    reveal_type(y != x)  # revealed: bool
+
+    reveal_type(x >= "abc")  # revealed: bool
+    reveal_type("abc" >= x)  # revealed: bool
+
+    reveal_type(x in "abc")  # revealed: bool
+    reveal_type("abc" in x)  # revealed: bool
+```
+
+#### Integers
+
+```py
+def get_int() -> int: ...
+
+x = get_int()
+
+if x != 1:
+    reveal_type(x)  # revealed: int & ~Literal[1]
+
+    reveal_type(x != 1)  # revealed: Literal[True]
+    reveal_type(x != 2)  # revealed: bool
+
+    reveal_type(x == 1)  # revealed: Literal[False]
+    reveal_type(x == 2)  # revealed: bool
+```
+
+### Identity comparisons
+
+```py
+class A: ...
+
+def get_object() -> object: ...
+
+o = object()
+
+a = A()
+n = None
+
+if o is not None:
+    reveal_type(o)  # revealed: object & ~None
+
+    reveal_type(o is n)  # revealed: Literal[False]
+    reveal_type(o is not n)  # revealed: Literal[True]
+```
+
+## Diagnostics
+
+### Unsupported operators for positive contributions
+
+Raise an error if any of the positive contributions to the intersection type are unsupported for the
+given operator:
+
+```py
+class Container:
+    def __contains__(self, x) -> bool: ...
+
+class NonContainer: ...
+
+def get_object() -> object: ...
+
+x = get_object()
+
+if isinstance(x, Container):
+    if isinstance(x, NonContainer):
+        reveal_type(x)  # revealed: Container & NonContainer
+
+        # error: [unsupported-operator] "Operator `in` is not supported for types `int` and `NonContainer`"
+        reveal_type(2 in x)  # revealed: bool
+```
+
+### Unsupported operators for negative contributions
+
+Do *not* raise an error if any of the negative contributions to the intersection type are
+unsupported for the given operator:
+
+```py
+class Container:
+    def __contains__(self, x) -> bool: ...
+
+class NonContainer: ...
+
+def get_object() -> object: ...
+
+x = get_object()
+
+if isinstance(x, Container):
+    if not isinstance(x, NonContainer):
+        reveal_type(x)  # revealed: Container & ~NonContainer
+
+        # No error here!
+        reveal_type(2 in x)  # revealed: bool
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/non_boolean_returns.md

@@ -5,9 +5,9 @@ Walking through examples:
 - `a = A() < B() < C()`
 
     1. `A() < B() and B() < C()` - split in N comparison
-    1. `A()` and `B()`              - evaluate outcome types
-    1. `bool` and `bool`            - evaluate truthiness
-    1. `A | B`                    - union of "first true" types
+    1. `A()` and `B()` - evaluate outcome types
+    1. `bool` and `bool` - evaluate truthiness
+    1. `A | B` - union of "first true" types
 
 - `b = 0 < 1 < A() < 3`
 

crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md

@@ -59,51 +59,51 @@ reveal_type(c >= d)  # revealed: Literal[True]
 
 ```py
 def bool_instance() -> bool: ...
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = (bool_instance(),)
 b = (int_instance(),)
 
-# TODO: All @Todo should be `bool`
-reveal_type(a == a)  # revealed: @Todo
-reveal_type(a != a)  # revealed: @Todo
-reveal_type(a < a)  # revealed: @Todo
-reveal_type(a <= a)  # revealed: @Todo
-reveal_type(a > a)  # revealed: @Todo
-reveal_type(a >= a)  # revealed: @Todo
+reveal_type(a == a)  # revealed: bool
+reveal_type(a != a)  # revealed: bool
+reveal_type(a < a)  # revealed: bool
+reveal_type(a <= a)  # revealed: bool
+reveal_type(a > a)  # revealed: bool
+reveal_type(a >= a)  # revealed: bool
 
-reveal_type(a == b)  # revealed: @Todo
-reveal_type(a != b)  # revealed: @Todo
-reveal_type(a < b)  # revealed: @Todo
-reveal_type(a <= b)  # revealed: @Todo
-reveal_type(a > b)  # revealed: @Todo
-reveal_type(a >= b)  # revealed: @Todo
+reveal_type(a == b)  # revealed: bool
+reveal_type(a != b)  # revealed: bool
+reveal_type(a < b)  # revealed: bool
+reveal_type(a <= b)  # revealed: bool
+reveal_type(a > b)  # revealed: bool
+reveal_type(a >= b)  # revealed: bool
 ```
 
 #### Comparison Unsupported
 
-If two tuples contain types that do not support comparison, the result may be `Unknown`.
-However, `==` and `!=` are exceptions and can still provide definite results.
+If two tuples contain types that do not support comparison, the result may be `Unknown`. However,
+`==` and `!=` are exceptions and can still provide definite results.
 
 ```py
 a = (1, 2)
 b = (1, "hello")
 
-# TODO: should be Literal[False]
-reveal_type(a == b)  # revealed: @Todo
+# TODO: should be Literal[False], once we implement (in)equality for mismatched literals
+reveal_type(a == b)  # revealed: bool
 
-# TODO: should be Literal[True]
-reveal_type(a != b)  # revealed: @Todo
+# TODO: should be Literal[True], once we implement (in)equality for mismatched literals
+reveal_type(a != b)  # revealed: bool
 
 # TODO: should be Unknown and add more informative diagnostics
-reveal_type(a < b)  # revealed: @Todo
-reveal_type(a <= b)  # revealed: @Todo
-reveal_type(a > b)  # revealed: @Todo
-reveal_type(a >= b)  # revealed: @Todo
+reveal_type(a < b)  # revealed: bool
+reveal_type(a <= b)  # revealed: bool
+reveal_type(a > b)  # revealed: bool
+reveal_type(a >= b)  # revealed: bool
 ```
 
-However, if the lexicographic comparison completes without reaching a point where str and int are compared,
-Python will still produce a result based on the prior elements.
+However, if the lexicographic comparison completes without reaching a point where str and int are
+compared, Python will still produce a result based on the prior elements.
 
 ```py path=short_circuit.py
 a = (1, 2)
@@ -145,13 +145,12 @@ class A:
 
 a = (A(), A())
 
-# TODO: All @Todo should be bool
-reveal_type(a == a)  # revealed: @Todo
-reveal_type(a != a)  # revealed: @Todo
-reveal_type(a < a)  # revealed: @Todo
-reveal_type(a <= a)  # revealed: @Todo
-reveal_type(a > a)  # revealed: @Todo
-reveal_type(a >= a)  # revealed: @Todo
+reveal_type(a == a)  # revealed: bool
+reveal_type(a != a)  # revealed: bool
+reveal_type(a < a)  # revealed: bool
+reveal_type(a <= a)  # revealed: bool
+reveal_type(a > a)  # revealed: bool
+reveal_type(a >= a)  # revealed: bool
 ```
 
 ### Membership Test Comparisons
@@ -159,7 +158,8 @@ reveal_type(a >= a)  # revealed: @Todo
 "Membership Test Comparisons" refers to the operators `in` and `not in`.
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = (1, 2)
 b = ((3, 4), (1, 2))
@@ -172,9 +172,8 @@ reveal_type(a not in b)  # revealed: Literal[False]
 reveal_type(a in c)  # revealed: Literal[False]
 reveal_type(a not in c)  # revealed: Literal[True]
 
-# TODO: All @Todo should be bool
-reveal_type(a in d)  # revealed: @Todo
-reveal_type(a not in d)  # revealed: @Todo
+reveal_type(a in d)  # revealed: bool
+reveal_type(a not in d)  # revealed: bool
 ```
 
 ### Identity Comparisons
@@ -189,10 +188,10 @@ c = (1, 2, 3)
 reveal_type(a is (1, 2))  # revealed: bool
 reveal_type(a is not (1, 2))  # revealed: bool
 
-# TODO: Update to Literal[False] once str == int comparison is implemented
-reveal_type(a is b)  # revealed: @Todo
-# TODO: Update to Literal[True] once str == int comparison is implemented
-reveal_type(a is not b)  # revealed: @Todo
+# TODO should be Literal[False] once we implement comparison of mismatched literal types
+reveal_type(a is b)  # revealed: bool
+# TODO should be Literal[True] once we implement comparison of mismatched literal types
+reveal_type(a is not b)  # revealed: bool
 
 reveal_type(a is c)  # revealed: Literal[False]
 reveal_type(a is not c)  # revealed: Literal[True]

crates/red_knot_python_semantic/resources/mdtest/comparison/unions.md

@@ -5,6 +5,10 @@
 Comparisons on union types need to consider all possible cases:
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 one_or_two = 1 if flag else 2
 
 reveal_type(one_or_two <= 2)  # revealed: Literal[True]
@@ -48,10 +52,14 @@ reveal_type(one_or_none is not None)  # revealed: bool
 
 ## Union on both sides of the comparison
 
-With unions on both sides, we need to consider the full cross product of
-options when building the resulting (union) type:
+With unions on both sides, we need to consider the full cross product of options when building the
+resulting (union) type:
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag_s, flag_l = bool_instance(), bool_instance()
 small = 1 if flag_s else 2
 large = 2 if flag_l else 3
 
@@ -64,11 +72,15 @@ reveal_type(small > large)  # revealed: Literal[False]
 
 ## Unsupported operations
 
-Make sure we emit a diagnostic if *any* of the possible comparisons is
-unsupported. For now, we fall back to `bool` for the result type instead of
-trying to infer something more precise from the other (supported) variants:
+Make sure we emit a diagnostic if *any* of the possible comparisons is unsupported. For now, we fall
+back to `bool` for the result type instead of trying to infer something more precise from the other
+(supported) variants:
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = [1, 2] if flag else 1
 
 result = 1 in x  # error: "Operator `in` is not supported"

crates/red_knot_python_semantic/resources/mdtest/comparison/unsupported.md

@@ -1,20 +1,28 @@
 # Comparison: Unsupported operators
 
 ```py
+def bool_instance() -> bool:
+    return True
+
 a = 1 in 7  # error: "Operator `in` is not supported for types `Literal[1]` and `Literal[7]`"
 reveal_type(a)  # revealed: bool
 
 b = 0 not in 10  # error: "Operator `not in` is not supported for types `Literal[0]` and `Literal[10]`"
 reveal_type(b)  # revealed: bool
 
-c = object() < 5  # error: "Operator `<` is not supported for types `object` and `int`"
-reveal_type(c)  # revealed: Unknown
+# TODO: should error, once operand type check is implemented
+# ("Operator `<` is not supported for types `object` and `int`")
+c = object() < 5
+# TODO: should be Unknown, once operand type check is implemented
+reveal_type(c)  # revealed: bool
 
-# TODO should error, need to check if __lt__ signature is valid for right operand
+# TODO: should error, once operand type check is implemented
+# ("Operator `<` is not supported for types `int` and `object`")
 d = 5 < object()
-# TODO: should be `Unknown`
+# TODO: should be Unknown, once operand type check is implemented
 reveal_type(d)  # revealed: bool
 
+flag = bool_instance()
 int_literal_or_str_literal = 1 if flag else "foo"
 # error: "Operator `in` is not supported for types `Literal[42]` and `Literal[1]`, in comparing `Literal[42]` with `Literal[1] | Literal["foo"]`"
 e = 42 in int_literal_or_str_literal
@@ -23,5 +31,6 @@ reveal_type(e)  # revealed: bool
 # TODO: should error, need to check if __lt__ signature is valid for right operand
 # error may be "Operator `<` is not supported for types `int` and `str`, in comparing `tuple[Literal[1], Literal[2]]` with `tuple[Literal[1], Literal["hello"]]`
 f = (1, 2) < (1, "hello")
-reveal_type(f)  # revealed: @Todo
+# TODO: should be Unknown, once operand type check is implemented
+reveal_type(f)  # revealed: bool
 ```

crates/red_knot_python_semantic/resources/mdtest/conditional/if_expression.md

@@ -3,6 +3,10 @@
 ## Simple if-expression
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else 2
 reveal_type(x)  # revealed: Literal[1, 2]
 ```
@@ -10,6 +14,10 @@ reveal_type(x)  # revealed: Literal[1, 2]
 ## If-expression with walrus operator
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 y = 0
 z = 0
 x = (y := 1) if flag else (z := 2)
@@ -21,6 +29,10 @@ reveal_type(z)  # revealed: Literal[0, 2]
 ## Nested if-expression
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 x = 1 if flag else 2 if flag2 else 3
 reveal_type(x)  # revealed: Literal[1, 2, 3]
 ```
@@ -28,6 +40,10 @@ reveal_type(x)  # revealed: Literal[1, 2, 3]
 ## None
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else None
 reveal_type(x)  # revealed: Literal[1] | None
 ```

crates/red_knot_python_semantic/resources/mdtest/conditional/if_statement.md

@@ -3,6 +3,10 @@
 ## Simple if
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 y = 1
 y = 2
 
@@ -15,6 +19,10 @@ reveal_type(y)  # revealed: Literal[2, 3]
 ## Simple if-elif-else
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 y = 1
 y = 2
 if flag:
@@ -28,13 +36,24 @@ else:
 x = y
 
 reveal_type(x)  # revealed: Literal[3, 4, 5]
-reveal_type(r)  # revealed: Unbound | Literal[2]
-reveal_type(s)  # revealed: Unbound | Literal[5]
+
+# revealed: Literal[2]
+# error: [possibly-unresolved-reference]
+reveal_type(r)
+
+# revealed: Literal[5]
+# error: [possibly-unresolved-reference]
+reveal_type(s)
 ```
 
 ## Single symbol across if-elif-else
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
+
 if flag:
     y = 1
 elif flag2:
@@ -47,6 +66,10 @@ reveal_type(y)  # revealed: Literal[1, 2, 3]
 ## if-elif-else without else assignment
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 y = 0
 if flag:
     y = 1
@@ -60,6 +83,10 @@ reveal_type(y)  # revealed: Literal[0, 1, 2]
 ## if-elif-else with intervening assignment
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 y = 0
 if flag:
     y = 1
@@ -74,6 +101,10 @@ reveal_type(y)  # revealed: Literal[0, 1, 2]
 ## Nested if statement
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 y = 0
 if flag:
     if flag2:
@@ -84,6 +115,10 @@ reveal_type(y)  # revealed: Literal[0, 1]
 ## if-elif without else
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 y = 1
 y = 2
 if flag:

crates/red_knot_python_semantic/resources/mdtest/conditional/match.md

@@ -21,7 +21,9 @@ match 0:
     case 2:
         y = 3
 
-reveal_type(y)  # revealed: Unbound | Literal[2, 3]
+# revealed: Literal[2, 3]
+# error: [possibly-unresolved-reference]
+reveal_type(y)
 ```
 
 ## Basic match

crates/red_knot_python_semantic/resources/mdtest/declaration/error.md

@@ -10,6 +10,10 @@ x: str  # error: [invalid-declaration] "Cannot declare type `str` for inferred t
 ## Incompatible declarations
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     x: str
 else:
@@ -20,6 +24,10 @@ x = 1  # error: [conflicting-declarations] "Conflicting declared types for `x`:
 ## Partial declarations
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     x: int
 x = 1  # error: [conflicting-declarations] "Conflicting declared types for `x`: Unknown, int"
@@ -28,6 +36,10 @@ x = 1  # error: [conflicting-declarations] "Conflicting declared types for `x`:
 ## Incompatible declarations with bad assignment
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     x: str
 else:

crates/red_knot_python_semantic/resources/mdtest/exception/basic.md

@@ -6,7 +6,7 @@
 import re
 
 try:
-    x
+    help()
 except NameError as e:
     reveal_type(e)  # revealed: NameError
 except re.error as f:
@@ -19,7 +19,7 @@ except re.error as f:
 from nonexistent_module import foo  # error: [unresolved-import]
 
 try:
-    x
+    help()
 except foo as e:
     reveal_type(foo)  # revealed: Unknown
     reveal_type(e)  # revealed: Unknown
@@ -31,7 +31,7 @@ except foo as e:
 EXCEPTIONS = (AttributeError, TypeError)
 
 try:
-    x
+    help()
 except (RuntimeError, OSError) as e:
     reveal_type(e)  # revealed: RuntimeError | OSError
 except EXCEPTIONS as f:
@@ -41,9 +41,14 @@ except EXCEPTIONS as f:
 ## Dynamic exception types
 
 ```py
-def foo(x: type[AttributeError], y: tuple[type[OSError], type[RuntimeError]], z: tuple[type[BaseException], ...]):
+# TODO: we should not emit these `call-possibly-unbound-method` errors for `tuple.__class_getitem__`
+def foo(
+    x: type[AttributeError],
+    y: tuple[type[OSError], type[RuntimeError]],  # error: [call-possibly-unbound-method]
+    z: tuple[type[BaseException], ...],  # error: [call-possibly-unbound-method]
+):
     try:
-        w
+        help()
     except x as e:
         # TODO: should be `AttributeError`
         reveal_type(e)  # revealed: @Todo

crates/red_knot_python_semantic/resources/mdtest/exception/control_flow.md

@@ -1,40 +1,33 @@
 # Control flow for exception handlers
 
-These tests assert that we understand the possible "definition states" (which
-symbols might or might not be defined) in the various branches of a
-`try`/`except`/`else`/`finally` block.
+These tests assert that we understand the possible "definition states" (which symbols might or might
+not be defined) in the various branches of a `try`/`except`/`else`/`finally` block.
 
-For a full writeup on the semantics of exception handlers,
-see [this document][1].
+For a full writeup on the semantics of exception handlers, see [this document][1].
 
-The tests throughout this Markdown document use functions with names starting
-with `could_raise_*` to mark definitions that might or might not succeed
-(as the function could raise an exception). A type checker must assume that any
-arbitrary function call could raise an exception in Python; this is just a
-naming convention used in these tests for clarity, and to future-proof the
-tests against possible future improvements whereby certain statements or
-expressions could potentially be inferred as being incapable of causing an
-exception to be raised.
+The tests throughout this Markdown document use functions with names starting with `could_raise_*`
+to mark definitions that might or might not succeed (as the function could raise an exception). A
+type checker must assume that any arbitrary function call could raise an exception in Python; this
+is just a naming convention used in these tests for clarity, and to future-proof the tests against
+possible future improvements whereby certain statements or expressions could potentially be inferred
+as being incapable of causing an exception to be raised.
 
 ## A single bare `except`
 
-Consider the following `try`/`except` block, with a single bare `except:`.
-There are different types for the variable `x` in the two branches of this
-block, and we can't determine which branch might have been taken from the
-perspective of code following this block. The inferred type after the block's
-conclusion is therefore the union of the type at the end of the `try` suite
-(`str`) and the type at the end of the `except` suite (`Literal[2]`).
+Consider the following `try`/`except` block, with a single bare `except:`. There are different types
+for the variable `x` in the two branches of this block, and we can't determine which branch might
+have been taken from the perspective of code following this block. The inferred type after the
+block's conclusion is therefore the union of the type at the end of the `try` suite (`str`) and the
+type at the end of the `except` suite (`Literal[2]`).
 
-*Within* the `except` suite, we must infer a union of all possible "definition
-states" we could have been in at any point during the `try` suite. This is
-because control flow could have jumped to the `except` suite without any of the
-`try`-suite definitions successfully completing, with only *some* of the
-`try`-suite definitions successfully completing, or indeed with *all* of them
-successfully completing. The type of `x` at the beginning of the `except` suite
-in this example is therefore `Literal[1] | str`, taking into account that we
-might have jumped to the `except` suite before the
-`x = could_raise_returns_str()` redefinition, but we *also* could have jumped
-to the `except` suite *after* that redefinition.
+*Within* the `except` suite, we must infer a union of all possible "definition states" we could have
+been in at any point during the `try` suite. This is because control flow could have jumped to the
+`except` suite without any of the `try`-suite definitions successfully completing, with only *some*
+of the `try`-suite definitions successfully completing, or indeed with *all* of them successfully
+completing. The type of `x` at the beginning of the `except` suite in this example is therefore
+`Literal[1] | str`, taking into account that we might have jumped to the `except` suite before the
+`x = could_raise_returns_str()` redefinition, but we *also* could have jumped to the `except` suite
+*after* that redefinition.
 
 ```py path=union_type_inferred.py
 def could_raise_returns_str() -> str:
@@ -54,9 +47,8 @@ except:
 reveal_type(x)  # revealed: str | Literal[2]
 ```
 
-If `x` has the same type at the end of both branches, however, the branches
-unify and `x` is not inferred as having a union type following the
-`try`/`except` block:
+If `x` has the same type at the end of both branches, however, the branches unify and `x` is not
+inferred as having a union type following the `try`/`except` block:
 
 ```py path=branches_unify_to_non_union_type.py
 def could_raise_returns_str() -> str:
@@ -74,13 +66,12 @@ reveal_type(x)  # revealed: str
 
 ## A non-bare `except`
 
-For simple `try`/`except` blocks, an `except TypeError:` handler has the same
-control flow semantics as an `except:` handler. An `except TypeError:` handler
-will not catch *all* exceptions: if this is the only handler, it opens up the
-possibility that an exception might occur that would not be handled. However,
-as described in [the document on exception-handling semantics][1], that would
-lead to termination of the scope. It's therefore irrelevant to consider this
-possibility when it comes to control-flow analysis.
+For simple `try`/`except` blocks, an `except TypeError:` handler has the same control flow semantics
+as an `except:` handler. An `except TypeError:` handler will not catch *all* exceptions: if this is
+the only handler, it opens up the possibility that an exception might occur that would not be
+handled. However, as described in [the document on exception-handling semantics][1], that would lead
+to termination of the scope. It's therefore irrelevant to consider this possibility when it comes to
+control-flow analysis.
 
 ```py
 def could_raise_returns_str() -> str:
@@ -102,11 +93,9 @@ reveal_type(x)  # revealed: str | Literal[2]
 
 ## Multiple `except` branches
 
-If the scope reaches the final `reveal_type` call in this example,
-either the `try`-block suite of statements was executed in its entirety,
-or exactly one `except` suite was executed in its entirety.
-The inferred type of `x` at this point is the union of the types at the end of
-the three suites:
+If the scope reaches the final `reveal_type` call in this example, either the `try`-block suite of
+statements was executed in its entirety, or exactly one `except` suite was executed in its entirety.
+The inferred type of `x` at this point is the union of the types at the end of the three suites:
 
 - At the end of `try`, `type(x) == str`
 - At the end of `except TypeError`, `x == 2`
@@ -136,11 +125,10 @@ reveal_type(x)  # revealed: str | Literal[2, 3]
 
 ## Exception handlers with `else` branches (but no `finally`)
 
-If we reach the `reveal_type` call at the end of this scope,
-either the `try` and `else` suites were both executed in their entireties,
-or the `except` suite was executed in its entirety. The type of `x` at this
-point is the union of the type at the end of the `else` suite and the type at
-the end of the `except` suite:
+If we reach the `reveal_type` call at the end of this scope, either the `try` and `else` suites were
+both executed in their entireties, or the `except` suite was executed in its entirety. The type of
+`x` at this point is the union of the type at the end of the `else` suite and the type at the end of
+the `except` suite:
 
 - At the end of `else`, `x == 3`
 - At the end of `except`, `x == 2`
@@ -167,10 +155,9 @@ else:
 reveal_type(x)  # revealed: Literal[2, 3]
 ```
 
-For a block that has multiple `except` branches and an `else` branch, the same
-principle applies. In order to reach the final `reveal_type` call,
-either exactly one of the `except` suites must have been executed in its
-entirety, or the `try` suite and the `else` suite must both have been executed
+For a block that has multiple `except` branches and an `else` branch, the same principle applies. In
+order to reach the final `reveal_type` call, either exactly one of the `except` suites must have
+been executed in its entirety, or the `try` suite and the `else` suite must both have been executed
 in their entireties:
 
 ```py
@@ -201,10 +188,9 @@ reveal_type(x)  # revealed: Literal[2, 3, 4]
 
 ## Exception handlers with `finally` branches (but no `except` branches)
 
-A `finally` suite is *always* executed. As such, if we reach the `reveal_type`
-call at the end of this example, we know that `x` *must* have been reassigned
-to `2` during the `finally` suite. The type of `x` at the end of the example is
-therefore `Literal[2]`:
+A `finally` suite is *always* executed. As such, if we reach the `reveal_type` call at the end of
+this example, we know that `x` *must* have been reassigned to `2` during the `finally` suite. The
+type of `x` at the end of the example is therefore `Literal[2]`:
 
 ```py path=redef_in_finally.py
 def could_raise_returns_str() -> str:
@@ -223,15 +209,13 @@ finally:
 reveal_type(x)  # revealed: Literal[2]
 ```
 
-If `x` was *not* redefined in the `finally` suite, however, things are somewhat
-more complicated. If we reach the final `reveal_type` call,
-unlike the state when we're visiting the `finally` suite,
-we know that the `try`-block suite ran to completion.
-This means that there are fewer possible states at this point than there were
-when we were inside the `finally` block.
+If `x` was *not* redefined in the `finally` suite, however, things are somewhat more complicated. If
+we reach the final `reveal_type` call, unlike the state when we're visiting the `finally` suite, we
+know that the `try`-block suite ran to completion. This means that there are fewer possible states
+at this point than there were when we were inside the `finally` block.
 
-(Our current model does *not* correctly infer the types *inside* `finally`
-suites, however; this is still a TODO item for us.)
+(Our current model does *not* correctly infer the types *inside* `finally` suites, however; this is
+still a TODO item for us.)
 
 ```py path=no_redef_in_finally.py
 def could_raise_returns_str() -> str:
@@ -252,18 +236,18 @@ reveal_type(x)  # revealed: str
 
 ## Combining an `except` branch with a `finally` branch
 
-As previously stated, we do not yet have accurate inference for types *inside*
-`finally` suites. When we do, however, we will have to take account of the
-following possibilities inside `finally` suites:
+As previously stated, we do not yet have accurate inference for types *inside* `finally` suites.
+When we do, however, we will have to take account of the following possibilities inside `finally`
+suites:
 
 - The `try` suite could have run to completion
-- Or we could have jumped from halfway through the `try` suite to an `except`
-    suite, and the `except` suite ran to completion
-- Or we could have jumped from halfway through the `try` suite straight to the
-    `finally` suite due to an unhandled exception
-- Or we could have jumped from halfway through the `try` suite to an
-    `except` suite, only for an exception raised in the `except` suite to cause
-    us to jump to the `finally` suite before the `except` suite ran to completion
+- Or we could have jumped from halfway through the `try` suite to an `except` suite, and the
+    `except` suite ran to completion
+- Or we could have jumped from halfway through the `try` suite straight to the `finally` suite due
+    to an unhandled exception
+- Or we could have jumped from halfway through the `try` suite to an `except` suite, only for an
+    exception raised in the `except` suite to cause us to jump to the `finally` suite before the
+    `except` suite ran to completion
 
 ```py path=redef_in_finally.py
 def could_raise_returns_str() -> str:
@@ -296,12 +280,11 @@ finally:
 reveal_type(x)  # revealed: Literal[2]
 ```
 
-Now for an example without a redefinition in the `finally` suite.
-As before, there *should* be fewer possibilities after completion of the
-`finally` suite than there were during the `finally` suite itself.
-(In some control-flow possibilities, some exceptions were merely *suspended*
-during the `finally` suite; these lead to the scope's termination following the
-conclusion of the `finally` suite.)
+Now for an example without a redefinition in the `finally` suite. As before, there *should* be fewer
+possibilities after completion of the `finally` suite than there were during the `finally` suite
+itself. (In some control-flow possibilities, some exceptions were merely *suspended* during the
+`finally` suite; these lead to the scope's termination following the conclusion of the `finally`
+suite.)
 
 ```py path=no_redef_in_finally.py
 def could_raise_returns_str() -> str:
@@ -377,9 +360,9 @@ reveal_type(x)  # revealed: str | bool | float
 
 ## Combining `except`, `else` and `finally` branches
 
-If the exception handler has an `else` branch, we must also take into account
-the possibility that control flow could have jumped to the `finally` suite from
-partway through the `else` suite due to an exception raised *there*.
+If the exception handler has an `else` branch, we must also take into account the possibility that
+control flow could have jumped to the `finally` suite from partway through the `else` suite due to
+an exception raised *there*.
 
 ```py path=single_except_branch.py
 def could_raise_returns_str() -> str:
@@ -479,15 +462,13 @@ reveal_type(x)  # revealed: bool | float | slice
 
 ## Nested `try`/`except` blocks
 
-It would take advanced analysis, which we are not yet capable of, to be able
-to determine that an exception handler always suppresses all exceptions. This
-is partly because it is possible for statements in `except`, `else` and
-`finally` suites to raise exceptions as well as statements in `try` suites.
-This means that if an exception handler is nested inside the `try` statement of
-an enclosing exception handler, it should (at least for now) be treated the
-same as any other node: as a suite containing statements that could possibly
-raise exceptions, which would lead to control flow jumping out of that suite
-prior to the suite running to completion.
+It would take advanced analysis, which we are not yet capable of, to be able to determine that an
+exception handler always suppresses all exceptions. This is partly because it is possible for
+statements in `except`, `else` and `finally` suites to raise exceptions as well as statements in
+`try` suites. This means that if an exception handler is nested inside the `try` statement of an
+enclosing exception handler, it should (at least for now) be treated the same as any other node: as
+a suite containing statements that could possibly raise exceptions, which would lead to control flow
+jumping out of that suite prior to the suite running to completion.
 
 ```py
 def could_raise_returns_str() -> str:
@@ -580,8 +561,8 @@ reveal_type(x)  # revealed: bytearray | Bar
 
 ## Nested scopes inside `try` blocks
 
-Shadowing a variable in an inner scope has no effect on type inference of the
-variable by that name in the outer scope:
+Shadowing a variable in an inner scope has no effect on type inference of the variable by that name
+in the outer scope:
 
 ```py
 def could_raise_returns_str() -> str:

crates/red_knot_python_semantic/resources/mdtest/exception/except_star.md

@@ -4,7 +4,7 @@
 
 ```py
 try:
-    x
+    help()
 except* BaseException as e:
     reveal_type(e)  # revealed: BaseExceptionGroup
 ```
@@ -13,7 +13,7 @@ except* BaseException as e:
 
 ```py
 try:
-    x
+    help()
 except* OSError as e:
     # TODO(Alex): more precise would be `ExceptionGroup[OSError]`
     reveal_type(e)  # revealed: BaseExceptionGroup
@@ -23,7 +23,7 @@ except* OSError as e:
 
 ```py
 try:
-    x
+    help()
 except* (TypeError, AttributeError) as e:
     # TODO(Alex): more precise would be `ExceptionGroup[TypeError | AttributeError]`.
     reveal_type(e)  # revealed: BaseExceptionGroup

crates/red_knot_python_semantic/resources/mdtest/exception/invalid_exception_syntax.md

@@ -0,0 +1,13 @@
+# Exception Handling
+
+## Invalid syntax
+
+```py
+from typing_extensions import reveal_type
+
+try:
+    print
+except as e:  # error: [invalid-syntax]
+    reveal_type(e)  # revealed: Unknown
+
+```

crates/red_knot_python_semantic/resources/mdtest/expression/if.md

@@ -0,0 +1,24 @@
+# If expression
+
+## Union
+
+```py
+def bool_instance() -> bool:
+    return True
+
+reveal_type(1 if bool_instance() else 2)  # revealed: Literal[1, 2]
+```
+
+## Statically known branches
+
+```py
+reveal_type(1 if True else 2)  # revealed: Literal[1]
+reveal_type(1 if "not empty" else 2)  # revealed: Literal[1]
+reveal_type(1 if (1,) else 2)  # revealed: Literal[1]
+reveal_type(1 if 1 else 2)  # revealed: Literal[1]
+
+reveal_type(1 if False else 2)  # revealed: Literal[2]
+reveal_type(1 if None else 2)  # revealed: Literal[2]
+reveal_type(1 if "" else 2)  # revealed: Literal[2]
+reveal_type(1 if 0 else 2)  # revealed: Literal[2]
+```

crates/red_knot_python_semantic/resources/mdtest/generics.md

@@ -6,16 +6,21 @@ Basic PEP 695 generics
 
 ```py
 class MyBox[T]:
+    # TODO: `T` is defined here
+    # error: [unresolved-reference] "Name `T` used when not defined"
     data: T
     box_model_number = 695
 
+    # TODO: `T` is defined here
+    # error: [unresolved-reference] "Name `T` used when not defined"
     def __init__(self, data: T):
         self.data = data
 
-# TODO not error (should be subscriptable)
-box: MyBox[int] = MyBox(5)  # error: [non-subscriptable]
-# TODO error differently (str and int don't unify)
-wrong_innards: MyBox[int] = MyBox("five")  # error: [non-subscriptable]
+box: MyBox[int] = MyBox(5)
+
+# TODO should emit a diagnostic here (str is not assignable to int)
+wrong_innards: MyBox[int] = MyBox("five")
+
 # TODO reveal int
 reveal_type(box.data)  # revealed: @Todo
 
@@ -26,13 +31,19 @@ reveal_type(MyBox.box_model_number)  # revealed: Literal[695]
 
 ```py
 class MyBox[T]:
+    # TODO: `T` is defined here
+    # error: [unresolved-reference] "Name `T` used when not defined"
     data: T
 
+    # TODO: `T` is defined here
+    # error: [unresolved-reference] "Name `T` used when not defined"
     def __init__(self, data: T):
         self.data = data
 
-# TODO not error on the subscripting
-class MySecureBox[T](MyBox[T]): ...  # error: [non-subscriptable]
+# TODO not error on the subscripting or the use of type param
+# error: [unresolved-reference] "Name `T` used when not defined"
+# error: [non-subscriptable]
+class MySecureBox[T](MyBox[T]): ...
 
 secure_box: MySecureBox[int] = MySecureBox(5)
 reveal_type(secure_box)  # revealed: MySecureBox
@@ -42,7 +53,8 @@ reveal_type(secure_box.data)  # revealed: @Todo
 
 ## Cyclical class definition
 
-In type stubs, classes can reference themselves in their base class definitions. For example, in `typeshed`, we have `class str(Sequence[str]): ...`.
+In type stubs, classes can reference themselves in their base class definitions. For example, in
+`typeshed`, we have `class str(Sequence[str]): ...`.
 
 This should hold true even with generics at play.
 

crates/red_knot_python_semantic/resources/mdtest/import/conditional.md

@@ -3,11 +3,21 @@
 ## Maybe unbound
 
 ```py path=maybe_unbound.py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     y = 3
-x = y
-reveal_type(x)  # revealed: Unbound | Literal[3]
-reveal_type(y)  # revealed: Unbound | Literal[3]
+
+x = y  # error: [possibly-unresolved-reference]
+
+# revealed: Literal[3]
+reveal_type(x)
+
+# revealed: Literal[3]
+# error: [possibly-unresolved-reference]
+reveal_type(y)
 ```
 
 ```py
@@ -20,11 +30,21 @@ reveal_type(y)  # revealed: Literal[3]
 ## Maybe unbound annotated
 
 ```py path=maybe_unbound_annotated.py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     y: int = 3
-x = y
-reveal_type(x)  # revealed: Unbound | Literal[3]
-reveal_type(y)  # revealed: Unbound | Literal[3]
+x = y  # error: [possibly-unresolved-reference]
+
+# revealed: Literal[3]
+reveal_type(x)
+
+# revealed: Literal[3]
+# error: [possibly-unresolved-reference]
+reveal_type(y)
 ```
 
 Importing an annotated name prefers the declared type over the inferred type:
@@ -36,6 +56,24 @@ reveal_type(x)  # revealed: Literal[3]
 reveal_type(y)  # revealed: int
 ```
 
+## Maybe undeclared
+
+Importing a possibly undeclared name still gives us its declared type:
+
+```py path=maybe_undeclared.py
+def bool_instance() -> bool:
+    return True
+
+if bool_instance():
+    x: int
+```
+
+```py
+from maybe_undeclared import x
+
+reveal_type(x)  # revealed: int
+```
+
 ## Reimport
 
 ```py path=c.py
@@ -43,6 +81,10 @@ def f(): ...
 ```
 
 ```py path=b.py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     from c import f
 else:
@@ -67,6 +109,10 @@ x: int
 ```
 
 ```py path=b.py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 if flag:
     from c import x
 else:

crates/red_knot_python_semantic/resources/mdtest/import/relative.md

@@ -102,7 +102,7 @@ reveal_type(X)  # revealed: Literal[42]
 ```
 
 ```py path=package/foo.py
-x
+x  # error: [unresolved-reference]
 ```
 
 ```py path=package/bar.py

crates/red_knot_python_semantic/resources/mdtest/literal/literal.md

@@ -0,0 +1,91 @@
+# Literal
+
+<https://typing.readthedocs.io/en/latest/spec/literal.html#literals>
+
+## Parameterization
+
+```py
+from typing import Literal
+from enum import Enum
+
+mode: Literal["w", "r"]
+mode2: Literal["w"] | Literal["r"]
+union_var: Literal[Literal[Literal[1, 2, 3], "foo"], 5, None]
+a1: Literal[26]
+a2: Literal[0x1A]
+a3: Literal[-4]
+a4: Literal["hello world"]
+a5: Literal[b"hello world"]
+a6: Literal[True]
+a7: Literal[None]
+a8: Literal[Literal[1]]
+a9: Literal[Literal["w"], Literal["r"], Literal[Literal["w+"]]]
+
+class Color(Enum):
+    RED = 0
+    GREEN = 1
+    BLUE = 2
+
+b1: Literal[Color.RED]
+
+def f():
+    reveal_type(mode)  # revealed: Literal["w", "r"]
+    reveal_type(mode2)  # revealed: Literal["w", "r"]
+    # TODO: should be revealed: Literal[1, 2, 3, "foo", 5] | None
+    reveal_type(union_var)  # revealed: Literal[1, 2, 3, 5] | Literal["foo"] | None
+    reveal_type(a1)  # revealed: Literal[26]
+    reveal_type(a2)  # revealed: Literal[26]
+    reveal_type(a3)  # revealed: Literal[-4]
+    reveal_type(a4)  # revealed: Literal["hello world"]
+    reveal_type(a5)  # revealed: Literal[b"hello world"]
+    reveal_type(a6)  # revealed: Literal[True]
+    reveal_type(a7)  # revealed: None
+    reveal_type(a8)  # revealed: Literal[1]
+    reveal_type(a9)  # revealed: Literal["w", "r", "w+"]
+    # TODO: This should be Color.RED
+    reveal_type(b1)  # revealed: Literal[0]
+
+# error: [invalid-literal-parameter]
+invalid1: Literal[3 + 4]
+# error: [invalid-literal-parameter]
+invalid2: Literal[4 + 3j]
+# error: [invalid-literal-parameter]
+invalid3: Literal[(3, 4)]
+invalid4: Literal[
+    1 + 2,  # error: [invalid-literal-parameter]
+    "foo",
+    hello,  # error: [invalid-literal-parameter]
+    (1, 2, 3),  # error: [invalid-literal-parameter]
+]
+```
+
+## Detecting Literal outside typing and typing_extensions
+
+Only Literal that is defined in typing and typing_extension modules is detected as the special
+Literal.
+
+```pyi path=other.pyi
+from typing import _SpecialForm
+
+Literal: _SpecialForm
+```
+
+```py
+from other import Literal
+
+a1: Literal[26]
+
+def f():
+    reveal_type(a1)  # revealed: @Todo
+```
+
+## Detecting typing_extensions.Literal
+
+```py
+from typing_extensions import Literal
+
+a1: Literal[26]
+
+def f():
+    reveal_type(a1)  # revealed: Literal[26]
+```

crates/red_knot_python_semantic/resources/mdtest/loops/async_for_loops.md

@@ -17,8 +17,10 @@ async def foo():
     async for x in Iterator():
         pass
 
-    # TODO
-    reveal_type(x)  # revealed: Unbound | @Todo
+    # TODO: should reveal `Unknown` because `__aiter__` is not defined
+    # revealed: @Todo
+    # error: [possibly-unresolved-reference]
+    reveal_type(x)
 ```
 
 ## Basic async for loop
@@ -37,5 +39,7 @@ async def foo():
     async for x in IntAsyncIterable():
         pass
 
-    reveal_type(x)  # revealed: Unbound | @Todo
+    # error: [possibly-unresolved-reference]
+    # revealed: @Todo
+    reveal_type(x)
 ```

crates/red_knot_python_semantic/resources/mdtest/loops/for_loop.md

@@ -14,7 +14,9 @@ class IntIterable:
 for x in IntIterable():
     pass
 
-reveal_type(x)  # revealed: Unbound | int
+# revealed: int
+# error: [possibly-unresolved-reference]
+reveal_type(x)
 ```
 
 ## With previous definition
@@ -85,7 +87,9 @@ class OldStyleIterable:
 for x in OldStyleIterable():
     pass
 
-reveal_type(x)  # revealed: Unbound | int
+# revealed: int
+# error: [possibly-unresolved-reference]
+reveal_type(x)
 ```
 
 ## With heterogeneous tuple
@@ -94,12 +98,19 @@ reveal_type(x)  # revealed: Unbound | int
 for x in (1, "a", b"foo"):
     pass
 
-reveal_type(x)  # revealed: Unbound | Literal[1] | Literal["a"] | Literal[b"foo"]
+# revealed: Literal[1] | Literal["a"] | Literal[b"foo"]
+# error: [possibly-unresolved-reference]
+reveal_type(x)
 ```
 
 ## With non-callable iterator
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 class NotIterable:
     if flag:
         __iter__ = 1
@@ -109,7 +120,9 @@ class NotIterable:
 for x in NotIterable():  # error: "Object of type `NotIterable` is not iterable"
     pass
 
-reveal_type(x)  # revealed: Unbound | Unknown
+# revealed: Unknown
+# error: [possibly-unresolved-reference]
+reveal_type(x)
 ```
 
 ## Invalid iterable
@@ -131,3 +144,140 @@ class NotIterable:
 for x in NotIterable():  # error: "Object of type `NotIterable` is not iterable"
     pass
 ```
+
+## Union type as iterable
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+class Test2:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+for x in Test() if flag else Test2():
+    reveal_type(x)  # revealed: int
+```
+
+## Union type as iterator
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class TestIter2:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter | TestIter2:
+        return TestIter()
+
+for x in Test():
+    reveal_type(x)  # revealed: int
+```
+
+## Union type as iterable and union type as iterator
+
+```py
+class TestIter:
+    def __next__(self) -> int | Exception:
+        return 42
+
+class TestIter2:
+    def __next__(self) -> str | tuple[int, int]:
+        return "42"
+
+class TestIter3:
+    def __next__(self) -> bytes:
+        return b"42"
+
+class TestIter4:
+    def __next__(self) -> memoryview:
+        return memoryview(b"42")
+
+class Test:
+    def __iter__(self) -> TestIter | TestIter2:
+        return TestIter()
+
+class Test2:
+    def __iter__(self) -> TestIter3 | TestIter4:
+        return TestIter3()
+
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+for x in Test() if flag else Test2():
+    reveal_type(x)  # revealed: int | Exception | str | tuple[int, int] | bytes | memoryview
+```
+
+## Union type as iterable where one union element has no `__iter__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+def coinflip() -> bool:
+    return True
+
+# error: [not-iterable] "Object of type `Test | Literal[42]` is not iterable because its `__iter__` method is possibly unbound"
+for x in Test() if coinflip() else 42:
+    reveal_type(x)  # revealed: int
+```
+
+## Union type as iterable where one union element has invalid `__iter__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+class Test2:
+    def __iter__(self) -> int:
+        return 42
+
+def coinflip() -> bool:
+    return True
+
+# error: "Object of type `Test | Test2` is not iterable"
+for x in Test() if coinflip() else Test2():
+    reveal_type(x)  # revealed: Unknown
+```
+
+## Union type as iterator where one union element has no `__next__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter | int:
+        return TestIter()
+
+# error: [not-iterable] "Object of type `Test` is not iterable"
+for x in Test():
+    reveal_type(x)  # revealed: Unknown
+```

crates/red_knot_python_semantic/resources/mdtest/loops/while_loop.md

@@ -3,6 +3,10 @@
 ## Basic While Loop
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1
 while flag:
     x = 2
@@ -13,6 +17,10 @@ reveal_type(x)  # revealed: Literal[1, 2]
 ## While with else (no break)
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1
 while flag:
     x = 2
@@ -26,6 +34,10 @@ reveal_type(x)  # revealed: Literal[3]
 ## While with Else (may break)
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag2 = bool_instance(), bool_instance()
 x = 1
 y = 0
 while flag:

crates/red_knot_python_semantic/resources/mdtest/metaclass.md

@@ -0,0 +1,196 @@
+## Default
+
+```py
+class M(type): ...
+
+reveal_type(M.__class__)  # revealed: Literal[type]
+```
+
+## `object`
+
+```py
+reveal_type(object.__class__)  # revealed: Literal[type]
+```
+
+## `type`
+
+```py
+reveal_type(type.__class__)  # revealed: Literal[type]
+```
+
+## Basic
+
+```py
+class M(type): ...
+class B(metaclass=M): ...
+
+reveal_type(B.__class__)  # revealed: Literal[M]
+```
+
+## Invalid metaclass
+
+A class which doesn't inherit `type` (and/or doesn't implement a custom `__new__` accepting the same
+arguments as `type.__new__`) isn't a valid metaclass.
+
+```py
+class M: ...
+class A(metaclass=M): ...
+
+# TODO: emit a diagnostic for the invalid metaclass
+reveal_type(A.__class__)  # revealed: Literal[M]
+```
+
+## Linear inheritance
+
+If a class is a subclass of a class with a custom metaclass, then the subclass will also have that
+metaclass.
+
+```py
+class M(type): ...
+class A(metaclass=M): ...
+class B(A): ...
+
+reveal_type(B.__class__)  # revealed: Literal[M]
+```
+
+## Conflict (1)
+
+The metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its
+bases. ("Strict subclass" is a synonym for "proper subclass"; a non-strict subclass can be a
+subclass or the class itself.)
+
+```py
+class M1(type): ...
+class M2(type): ...
+class A(metaclass=M1): ...
+class B(metaclass=M2): ...
+
+# error: [conflicting-metaclass] "The metaclass of a derived class (`C`) must be a subclass of the metaclasses of all its bases, but `M1` (metaclass of base class `A`) and `M2` (metaclass of base class `B`) have no subclass relationship"
+class C(A, B): ...
+
+reveal_type(C.__class__)  # revealed: Unknown
+```
+
+## Conflict (2)
+
+The metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its
+bases. ("Strict subclass" is a synonym for "proper subclass"; a non-strict subclass can be a
+subclass or the class itself.)
+
+```py
+class M1(type): ...
+class M2(type): ...
+class A(metaclass=M1): ...
+
+# error: [conflicting-metaclass] "The metaclass of a derived class (`B`) must be a subclass of the metaclasses of all its bases, but `M2` (metaclass of `B`) and `M1` (metaclass of base class `A`) have no subclass relationship"
+class B(A, metaclass=M2): ...
+
+reveal_type(B.__class__)  # revealed: Unknown
+```
+
+## Common metaclass
+
+A class has two explicit bases, both of which have the same metaclass.
+
+```py
+class M(type): ...
+class A(metaclass=M): ...
+class B(metaclass=M): ...
+class C(A, B): ...
+
+reveal_type(C.__class__)  # revealed: Literal[M]
+```
+
+## Metaclass metaclass
+
+A class has an explicit base with a custom metaclass. That metaclass itself has a custom metaclass.
+
+```py
+class M1(type): ...
+class M2(type, metaclass=M1): ...
+class M3(M2): ...
+class A(metaclass=M3): ...
+class B(A): ...
+
+reveal_type(A.__class__)  # revealed: Literal[M3]
+```
+
+## Diamond inheritance
+
+```py
+class M(type): ...
+class M1(M): ...
+class M2(M): ...
+class M12(M1, M2): ...
+class A(metaclass=M1): ...
+class B(metaclass=M2): ...
+class C(metaclass=M12): ...
+
+# error: [conflicting-metaclass] "The metaclass of a derived class (`D`) must be a subclass of the metaclasses of all its bases, but `M1` (metaclass of base class `A`) and `M2` (metaclass of base class `B`) have no subclass relationship"
+class D(A, B, C): ...
+
+reveal_type(D.__class__)  # revealed: Unknown
+```
+
+## Unknown
+
+```py
+from nonexistent_module import UnknownClass  # error: [unresolved-import]
+
+class C(UnknownClass): ...
+
+# TODO: should be `type[type] & Unknown`
+reveal_type(C.__class__)  # revealed: Literal[type]
+
+class M(type): ...
+class A(metaclass=M): ...
+class B(A, UnknownClass): ...
+
+# TODO: should be `type[M] & Unknown`
+reveal_type(B.__class__)  # revealed: Literal[M]
+```
+
+## Duplicate
+
+```py
+class M(type): ...
+class A(metaclass=M): ...
+class B(A, A): ...  # error: [duplicate-base] "Duplicate base class `A`"
+
+reveal_type(B.__class__)  # revealed: Literal[M]
+```
+
+## Non-class
+
+When a class has an explicit `metaclass` that is not a class, but is a callable that accepts
+`type.__new__` arguments, we should return the meta type of its return type.
+
+```py
+def f(*args, **kwargs) -> int: ...
+
+class A(metaclass=f): ...
+
+# TODO should be `type[int]`
+reveal_type(A.__class__)  # revealed: @Todo
+```
+
+## Cyclic
+
+Retrieving the metaclass of a cyclically defined class should not cause an infinite loop.
+
+```py path=a.pyi
+class A(B): ...  # error: [cyclic-class-def]
+class B(C): ...  # error: [cyclic-class-def]
+class C(A): ...  # error: [cyclic-class-def]
+
+reveal_type(A.__class__)  # revealed: Unknown
+```
+
+## PEP 695 generic
+
+```py
+class M(type): ...
+class A[T: str](metaclass=M): ...
+
+reveal_type(A.__class__)  # revealed: Literal[M]
+```

crates/red_knot_python_semantic/resources/mdtest/mro.md

@@ -0,0 +1,409 @@
+# Method Resolution Order tests
+
+Tests that assert that we can infer the correct type for a class's `__mro__` attribute.
+
+This attribute is rarely accessed directly at runtime. However, it's extremely important for *us* to
+know the precise possible values of a class's Method Resolution Order, or we won't be able to infer
+the correct type of attributes accessed from instances.
+
+For documentation on method resolution orders, see:
+
+- <https://docs.python.org/3/glossary.html#term-method-resolution-order>
+- <https://docs.python.org/3/howto/mro.html#python-2-3-mro>
+
+## No bases
+
+```py
+class C: ...
+
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[object]]
+```
+
+## The special case: `object` itself
+
+```py
+reveal_type(object.__mro__)  # revealed: tuple[Literal[object]]
+```
+
+## Explicit inheritance from `object`
+
+```py
+class C(object): ...
+
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[object]]
+```
+
+## Explicit inheritance from non-`object` single base
+
+```py
+class A: ...
+class B(A): ...
+
+reveal_type(B.__mro__)  # revealed: tuple[Literal[B], Literal[A], Literal[object]]
+```
+
+## Linearization of multiple bases
+
+```py
+class A: ...
+class B: ...
+class C(A, B): ...
+
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[A], Literal[B], Literal[object]]
+```
+
+## Complex diamond inheritance (1)
+
+This is "ex_2" from <https://docs.python.org/3/howto/mro.html#the-end>
+
+```py
+class O: ...
+class X(O): ...
+class Y(O): ...
+class A(X, Y): ...
+class B(Y, X): ...
+
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Literal[X], Literal[Y], Literal[O], Literal[object]]
+reveal_type(B.__mro__)  # revealed: tuple[Literal[B], Literal[Y], Literal[X], Literal[O], Literal[object]]
+```
+
+## Complex diamond inheritance (2)
+
+This is "ex_5" from <https://docs.python.org/3/howto/mro.html#the-end>
+
+```py
+class O: ...
+class F(O): ...
+class E(O): ...
+class D(O): ...
+class C(D, F): ...
+class B(D, E): ...
+class A(B, C): ...
+
+# revealed: tuple[Literal[C], Literal[D], Literal[F], Literal[O], Literal[object]]
+reveal_type(C.__mro__)
+# revealed: tuple[Literal[B], Literal[D], Literal[E], Literal[O], Literal[object]]
+reveal_type(B.__mro__)
+# revealed: tuple[Literal[A], Literal[B], Literal[C], Literal[D], Literal[E], Literal[F], Literal[O], Literal[object]]
+reveal_type(A.__mro__)
+```
+
+## Complex diamond inheritance (3)
+
+This is "ex_6" from <https://docs.python.org/3/howto/mro.html#the-end>
+
+```py
+class O: ...
+class F(O): ...
+class E(O): ...
+class D(O): ...
+class C(D, F): ...
+class B(E, D): ...
+class A(B, C): ...
+
+# revealed: tuple[Literal[C], Literal[D], Literal[F], Literal[O], Literal[object]]
+reveal_type(C.__mro__)
+# revealed: tuple[Literal[B], Literal[E], Literal[D], Literal[O], Literal[object]]
+reveal_type(B.__mro__)
+# revealed: tuple[Literal[A], Literal[B], Literal[E], Literal[C], Literal[D], Literal[F], Literal[O], Literal[object]]
+reveal_type(A.__mro__)
+```
+
+## Complex diamond inheritance (4)
+
+This is "ex_9" from <https://docs.python.org/3/howto/mro.html#the-end>
+
+```py
+class O: ...
+class A(O): ...
+class B(O): ...
+class C(O): ...
+class D(O): ...
+class E(O): ...
+class K1(A, B, C): ...
+class K2(D, B, E): ...
+class K3(D, A): ...
+class Z(K1, K2, K3): ...
+
+# revealed: tuple[Literal[K1], Literal[A], Literal[B], Literal[C], Literal[O], Literal[object]]
+reveal_type(K1.__mro__)
+# revealed: tuple[Literal[K2], Literal[D], Literal[B], Literal[E], Literal[O], Literal[object]]
+reveal_type(K2.__mro__)
+# revealed: tuple[Literal[K3], Literal[D], Literal[A], Literal[O], Literal[object]]
+reveal_type(K3.__mro__)
+# revealed: tuple[Literal[Z], Literal[K1], Literal[K2], Literal[K3], Literal[D], Literal[A], Literal[B], Literal[C], Literal[E], Literal[O], Literal[object]]
+reveal_type(Z.__mro__)
+```
+
+## Inheritance from `Unknown`
+
+```py
+from does_not_exist import DoesNotExist  # error: [unresolved-import]
+
+class A(DoesNotExist): ...
+class B: ...
+class C: ...
+class D(A, B, C): ...
+class E(B, C): ...
+class F(E, A): ...
+
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Unknown, Literal[object]]
+reveal_type(D.__mro__)  # revealed: tuple[Literal[D], Literal[A], Unknown, Literal[B], Literal[C], Literal[object]]
+reveal_type(E.__mro__)  # revealed: tuple[Literal[E], Literal[B], Literal[C], Literal[object]]
+reveal_type(F.__mro__)  # revealed: tuple[Literal[F], Literal[E], Literal[B], Literal[C], Literal[A], Unknown, Literal[object]]
+```
+
+## `__bases__` lists that cause errors at runtime
+
+If the class's `__bases__` cause an exception to be raised at runtime and therefore the class
+creation to fail, we infer the class's `__mro__` as being `[<class>, Unknown, object]`:
+
+```py
+# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Foo` with bases list `[<class 'object'>, <class 'int'>]`"
+class Foo(object, int): ...
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+
+class Bar(Foo): ...
+
+reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Literal[Foo], Unknown, Literal[object]]
+
+# This is the `TypeError` at the bottom of "ex_2"
+# in the examples at <https://docs.python.org/3/howto/mro.html#the-end>
+class O: ...
+class X(O): ...
+class Y(O): ...
+class A(X, Y): ...
+class B(Y, X): ...
+
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Literal[X], Literal[Y], Literal[O], Literal[object]]
+reveal_type(B.__mro__)  # revealed: tuple[Literal[B], Literal[Y], Literal[X], Literal[O], Literal[object]]
+
+# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Z` with bases list `[<class 'A'>, <class 'B'>]`"
+class Z(A, B): ...
+
+reveal_type(Z.__mro__)  # revealed: tuple[Literal[Z], Unknown, Literal[object]]
+
+class AA(Z): ...
+
+reveal_type(AA.__mro__)  # revealed: tuple[Literal[AA], Literal[Z], Unknown, Literal[object]]
+```
+
+## `__bases__` includes a `Union`
+
+We don't support union types in a class's bases; a base must resolve to a single `ClassLiteralType`.
+If we find a union type in a class's bases, we infer the class's `__mro__` as being
+`[<class>, Unknown, object]`, the same as for MROs that cause errors at runtime.
+
+```py
+def returns_bool() -> bool:
+    return True
+
+class A: ...
+class B: ...
+
+if returns_bool():
+    x = A
+else:
+    x = B
+
+reveal_type(x)  # revealed: Literal[A, B]
+
+# error: 11 [invalid-base] "Invalid class base with type `Literal[A, B]` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+class Foo(x): ...
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+```
+
+## `__bases__` includes multiple `Union`s
+
+```py
+def returns_bool() -> bool:
+    return True
+
+class A: ...
+class B: ...
+class C: ...
+class D: ...
+
+if returns_bool():
+    x = A
+else:
+    x = B
+
+if returns_bool():
+    y = C
+else:
+    y = D
+
+reveal_type(x)  # revealed: Literal[A, B]
+reveal_type(y)  # revealed: Literal[C, D]
+
+# error: 11 [invalid-base] "Invalid class base with type `Literal[A, B]` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+# error: 14 [invalid-base] "Invalid class base with type `Literal[C, D]` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+class Foo(x, y): ...
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+```
+
+## `__bases__` lists that cause errors... now with `Union`s
+
+```py
+def returns_bool() -> bool:
+    return True
+
+class O: ...
+class X(O): ...
+class Y(O): ...
+
+if bool():
+    foo = Y
+else:
+    foo = object
+
+# error: 21 [invalid-base] "Invalid class base with type `Literal[Y, object]` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+class PossibleError(foo, X): ...
+
+reveal_type(PossibleError.__mro__)  # revealed: tuple[Literal[PossibleError], Unknown, Literal[object]]
+
+class A(X, Y): ...
+
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Literal[X], Literal[Y], Literal[O], Literal[object]]
+
+if returns_bool():
+    class B(X, Y): ...
+
+else:
+    class B(Y, X): ...
+
+# revealed: tuple[Literal[B], Literal[X], Literal[Y], Literal[O], Literal[object]] | tuple[Literal[B], Literal[Y], Literal[X], Literal[O], Literal[object]]
+reveal_type(B.__mro__)
+
+# error: 12 [invalid-base] "Invalid class base with type `Literal[B, B]` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+class Z(A, B): ...
+
+reveal_type(Z.__mro__)  # revealed: tuple[Literal[Z], Unknown, Literal[object]]
+```
+
+## `__bases__` lists with duplicate bases
+
+```py
+class Foo(str, str): ...  # error: 16 [duplicate-base] "Duplicate base class `str`"
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+
+class Spam: ...
+class Eggs: ...
+class Ham(
+    Spam,
+    Eggs,
+    Spam,  # error: [duplicate-base] "Duplicate base class `Spam`"
+    Eggs,  # error: [duplicate-base] "Duplicate base class `Eggs`"
+): ...
+
+reveal_type(Ham.__mro__)  # revealed: tuple[Literal[Ham], Unknown, Literal[object]]
+
+class Mushrooms: ...
+class Omelette(Spam, Eggs, Mushrooms, Mushrooms): ...  # error: [duplicate-base]
+
+reveal_type(Omelette.__mro__)  # revealed: tuple[Literal[Omelette], Unknown, Literal[object]]
+```
+
+## `__bases__` lists with duplicate `Unknown` bases
+
+```py
+# error: [unresolved-import]
+# error: [unresolved-import]
+from does_not_exist import unknown_object_1, unknown_object_2
+
+reveal_type(unknown_object_1)  # revealed: Unknown
+reveal_type(unknown_object_2)  # revealed: Unknown
+
+# We *should* emit an error here to warn the user that we have no idea
+# what the MRO of this class should really be.
+# However, we don't complain about "duplicate base classes" here,
+# even though two classes are both inferred as being `Unknown`.
+#
+# (TODO: should we revisit this? Does it violate the gradual guarantee?
+# Should we just silently infer `[Foo, Unknown, object]` as the MRO here
+# without emitting any error at all? Not sure...)
+#
+# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Foo` with bases list `[Unknown, Unknown]`"
+class Foo(unknown_object_1, unknown_object_2): ...
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+```
+
+## Unrelated objects inferred as `Any`/`Unknown` do not have special `__mro__` attributes
+
+```py
+from does_not_exist import unknown_object  # error: [unresolved-import]
+
+reveal_type(unknown_object)  # revealed: Unknown
+reveal_type(unknown_object.__mro__)  # revealed: Unknown
+```
+
+## Classes that inherit from themselves
+
+These are invalid, but we need to be able to handle them gracefully without panicking.
+
+```py path=a.pyi
+class Foo(Foo): ...  # error: [cyclic-class-def]
+
+reveal_type(Foo)  # revealed: Literal[Foo]
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+
+class Bar: ...
+class Baz: ...
+class Boz(Bar, Baz, Boz): ...  # error: [cyclic-class-def]
+
+reveal_type(Boz)  # revealed: Literal[Boz]
+reveal_type(Boz.__mro__)  # revealed: tuple[Literal[Boz], Unknown, Literal[object]]
+```
+
+## Classes with indirect cycles in their MROs
+
+These are similarly unlikely, but we still shouldn't crash:
+
+```py path=a.pyi
+class Foo(Bar): ...  # error: [cyclic-class-def]
+class Bar(Baz): ...  # error: [cyclic-class-def]
+class Baz(Foo): ...  # error: [cyclic-class-def]
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
+reveal_type(Baz.__mro__)  # revealed: tuple[Literal[Baz], Unknown, Literal[object]]
+```
+
+## Classes with cycles in their MROs, and multiple inheritance
+
+```py path=a.pyi
+class Spam: ...
+class Foo(Bar): ...  # error: [cyclic-class-def]
+class Bar(Baz): ...  # error: [cyclic-class-def]
+class Baz(Foo, Spam): ...  # error: [cyclic-class-def]
+
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
+reveal_type(Baz.__mro__)  # revealed: tuple[Literal[Baz], Unknown, Literal[object]]
+```
+
+## Classes with cycles in their MRO, and a sub-graph
+
+```py path=a.pyi
+class FooCycle(BarCycle): ...  # error: [cyclic-class-def]
+class Foo: ...
+class BarCycle(FooCycle): ...  # error: [cyclic-class-def]
+class Bar(Foo): ...
+
+# TODO: can we avoid emitting the errors for these?
+# The classes have cyclic superclasses,
+# but are not themselves cyclic...
+class Baz(Bar, BarCycle): ...  # error: [cyclic-class-def]
+class Spam(Baz): ...  # error: [cyclic-class-def]
+
+reveal_type(FooCycle.__mro__)  # revealed: tuple[Literal[FooCycle], Unknown, Literal[object]]
+reveal_type(BarCycle.__mro__)  # revealed: tuple[Literal[BarCycle], Unknown, Literal[object]]
+reveal_type(Baz.__mro__)  # revealed: tuple[Literal[Baz], Unknown, Literal[object]]
+reveal_type(Spam.__mro__)  # revealed: tuple[Literal[Spam], Unknown, Literal[object]]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/boolean.md

@@ -0,0 +1,93 @@
+# Narrowing in boolean expressions
+
+In `or` expressions, the right-hand side is evaluated only if the left-hand side is **falsy**. So
+when the right-hand side is evaluated, we know the left side has failed.
+
+Similarly, in `and` expressions, the right-hand side is evaluated only if the left-hand side is
+**truthy**. So when the right-hand side is evaluated, we know the left side has succeeded.
+
+## Narrowing in `or`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None = A() if bool_instance() else None
+
+isinstance(x, A) or reveal_type(x)  # revealed: None
+x is None or reveal_type(x)  # revealed: A
+reveal_type(x)  # revealed: A | None
+```
+
+## Narrowing in `and`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None = A() if bool_instance() else None
+
+isinstance(x, A) and reveal_type(x)  # revealed: A
+x is None and reveal_type(x)  # revealed: None
+reveal_type(x)  # revealed: A | None
+```
+
+## Multiple `and` arms
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None = A() if bool_instance() else None
+
+bool_instance() and isinstance(x, A) and reveal_type(x)  # revealed: A
+isinstance(x, A) and bool_instance() and reveal_type(x)  # revealed: A
+reveal_type(x) and isinstance(x, A) and bool_instance()  # revealed: A | None
+```
+
+## Multiple `or` arms
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None = A() if bool_instance() else None
+
+bool_instance() or isinstance(x, A) or reveal_type(x)  # revealed: None
+isinstance(x, A) or bool_instance() or reveal_type(x)  # revealed: None
+reveal_type(x) or isinstance(x, A) or bool_instance()  # revealed: A | None
+```
+
+## Multiple predicates
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None | Literal[1] = A() if bool_instance() else None if bool_instance() else 1
+
+x is None or isinstance(x, A) or reveal_type(x)  # revealed: Literal[1]
+```
+
+## Mix of `and` and `or`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x: A | None | Literal[1] = A() if bool_instance() else None if bool_instance() else 1
+
+isinstance(x, A) or x is not None and reveal_type(x)  # revealed: Literal[1]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_boolean.md

@@ -0,0 +1,282 @@
+# Narrowing for conditionals with boolean expressions
+
+## Narrowing in `and` conditional
+
+```py
+class A: ...
+class B: ...
+
+def instance() -> A | B:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) and isinstance(x, B):
+    reveal_type(x)  # revealed:  A & B
+else:
+    reveal_type(x)  # revealed:  B & ~A | A & ~B
+```
+
+## Arms might not add narrowing constraints
+
+```py
+class A: ...
+class B: ...
+
+def bool_instance() -> bool:
+    return True
+
+def instance() -> A | B:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) and bool_instance():
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: A | B
+
+if bool_instance() and isinstance(x, A):
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: A | B
+
+reveal_type(x)  # revealed: A | B
+```
+
+## Statically known arms
+
+```py
+class A: ...
+class B: ...
+
+def instance() -> A | B:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) and True:
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: B & ~A
+
+if True and isinstance(x, A):
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: B & ~A
+
+if False and isinstance(x, A):
+    # TODO: should emit an `unreachable code` diagnostic
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: A | B
+
+if False or isinstance(x, A):
+    reveal_type(x)  # revealed: A
+else:
+    reveal_type(x)  # revealed: B & ~A
+
+if True or isinstance(x, A):
+    reveal_type(x)  # revealed: A | B
+else:
+    # TODO: should emit an `unreachable code` diagnostic
+    reveal_type(x)  # revealed: B & ~A
+
+reveal_type(x)  # revealed: A | B
+```
+
+## The type of multiple symbols can be narrowed down
+
+```py
+class A: ...
+class B: ...
+
+def instance() -> A | B:
+    return A()
+
+x = instance()
+y = instance()
+
+if isinstance(x, A) and isinstance(y, B):
+    reveal_type(x)  # revealed: A
+    reveal_type(y)  # revealed: B
+else:
+    # No narrowing: Only-one or both checks might have failed
+    reveal_type(x)  # revealed: A | B
+    reveal_type(y)  # revealed: A | B
+
+reveal_type(x)  # revealed: A | B
+reveal_type(y)  # revealed: A | B
+```
+
+## Narrowing in `or` conditional
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) or isinstance(x, B):
+    reveal_type(x)  # revealed:  A | B
+else:
+    reveal_type(x)  # revealed:  C & ~A & ~B
+```
+
+## In `or`, all arms should add constraint in order to narrow
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+def bool_instance() -> bool:
+    return True
+
+x = instance()
+
+if isinstance(x, A) or isinstance(x, B) or bool_instance():
+    reveal_type(x)  # revealed:  A | B | C
+else:
+    reveal_type(x)  # revealed:  C & ~A & ~B
+```
+
+## in `or`, all arms should narrow the same set of symbols
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+y = instance()
+
+if isinstance(x, A) or isinstance(y, A):
+    # The predicate might be satisfied by the right side, so the type of `x` can’t be narrowed down here.
+    reveal_type(x)  # revealed:  A | B | C
+    # The same for `y`
+    reveal_type(y)  # revealed:  A | B | C
+else:
+    reveal_type(x)  # revealed:  B & ~A | C & ~A
+    reveal_type(y)  # revealed:  B & ~A | C & ~A
+
+if (isinstance(x, A) and isinstance(y, A)) or (isinstance(x, B) and isinstance(y, B)):
+    # Here, types of `x` and `y` can be narrowd since all `or` arms constraint them.
+    reveal_type(x)  # revealed:  A | B
+    reveal_type(y)  # revealed:  A | B
+else:
+    reveal_type(x)  # revealed:  A | B | C
+    reveal_type(y)  # revealed:  A | B | C
+```
+
+## mixing `and` and `not`
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+
+if isinstance(x, B) and not isinstance(x, C):
+    reveal_type(x)  # revealed:  B & ~C
+else:
+    # ~(B & ~C) -> ~B | C -> (A & ~B) | (C & ~B) | C -> (A & ~B) | C
+    reveal_type(x)  # revealed: A & ~B | C
+```
+
+## mixing `or` and `not`
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+
+if isinstance(x, B) or not isinstance(x, C):
+    reveal_type(x)  # revealed: B | A & ~C
+else:
+    reveal_type(x)  # revealed: C & ~B
+```
+
+## `or` with nested `and`
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) or (isinstance(x, B) and not isinstance(x, C)):
+    reveal_type(x)  # revealed:  A | B & ~C
+else:
+    # ~(A | (B & ~C)) -> ~A & ~(B & ~C) -> ~A & (~B | C) -> (~A & C) | (~A ~ B)
+    reveal_type(x)  # revealed:  C & ~A
+```
+
+## `and` with nested `or`
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def instance() -> A | B | C:
+    return A()
+
+x = instance()
+
+if isinstance(x, A) and (isinstance(x, B) or not isinstance(x, C)):
+    # A & (B | ~C) -> (A & B) | (A & ~C)
+    reveal_type(x)  # revealed:  A & B | A & ~C
+else:
+    # ~((A & B) | (A & ~C)) ->
+    # ~(A & B) & ~(A & ~C) ->
+    # (~A | ~B) & (~A | C) ->
+    # [(~A | ~B) & ~A] | [(~A | ~B) & C] ->
+    # ~A | (~A & C) | (~B & C) ->
+    # ~A | (C & ~B) ->
+    # ~A | (C & ~B)  The positive side of ~A is  A | B | C ->
+    reveal_type(x)  # revealed:  B & ~A | C & ~A | C & ~B
+```
+
+## Boolean expression internal narrowing
+
+```py
+def optional_string() -> str | None:
+    return None
+
+x = optional_string()
+y = optional_string()
+
+if x is None and y is not x:
+    reveal_type(y)  # revealed: str
+
+# Neither of the conditions alone is sufficient for narrowing y's type:
+if x is None:
+    reveal_type(y)  # revealed: str | None
+
+if y is not x:
+    reveal_type(y)  # revealed: str | None
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_elif_else.md

@@ -0,0 +1,57 @@
+# Narrowing for conditionals with elif and else
+
+## Positive contributions become negative in elif-else blocks
+
+```py
+def int_instance() -> int:
+    return 42
+
+x = int_instance()
+
+if x == 1:
+    # cannot narrow; could be a subclass of `int`
+    reveal_type(x)  # revealed: int
+elif x == 2:
+    reveal_type(x)  # revealed: int & ~Literal[1]
+elif x != 3:
+    reveal_type(x)  # revealed: int & ~Literal[1] & ~Literal[2] & ~Literal[3]
+```
+
+## Positive contributions become negative in elif-else blocks, with simplification
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x == 1:
+    # TODO should be Literal[1]
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+elif x == 2:
+    # TODO should be Literal[2]
+    reveal_type(x)  # revealed: Literal[2, 3]
+else:
+    reveal_type(x)  # revealed: Literal[3]
+```
+
+## Multiple negative contributions using elif, with simplification
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x != 1:
+    reveal_type(x)  # revealed: Literal[2, 3]
+elif x != 2:
+    # TODO should be `Literal[1]`
+    reveal_type(x)  # revealed: Literal[1, 3]
+elif x == 3:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+else:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is.md

@@ -3,10 +3,16 @@
 ## `is None`
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = None if flag else 1
 
 if x is None:
     reveal_type(x)  # revealed: None
+else:
+    reveal_type(x)  # revealed: Literal[1]
 
 reveal_type(x)  # revealed: None | Literal[1]
 ```
@@ -14,6 +20,11 @@ reveal_type(x)  # revealed: None | Literal[1]
 ## `is` for other types
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 class A: ...
 
 x = A()
@@ -21,6 +32,8 @@ y = x if flag else None
 
 if y is x:
     reveal_type(y)  # revealed: A
+else:
+    reveal_type(y)  # revealed: A | None
 
 reveal_type(y)  # revealed: A | None
 ```
@@ -28,6 +41,10 @@ reveal_type(y)  # revealed: A | None
 ## `is` in chained comparisons
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+x_flag, y_flag = bool_instance(), bool_instance()
 x = True if x_flag else False
 y = True if y_flag else False
 
@@ -37,4 +54,26 @@ reveal_type(y)  # revealed: bool
 if y is x is False:  # Interpreted as `(y is x) and (x is False)`
     reveal_type(x)  # revealed: Literal[False]
     reveal_type(y)  # revealed: bool
+else:
+    # The negation of the clause above is (y is not x) or (x is not False)
+    # So we can't narrow the type of x or y here, because each arm of the `or` could be true
+    reveal_type(x)  # revealed: bool
+    reveal_type(y)  # revealed: bool
+```
+
+## `is` in elif clause
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = None if bool_instance() else (1 if bool_instance() else True)
+
+reveal_type(x)  # revealed: None | Literal[1] | Literal[True]
+if x is None:
+    reveal_type(x)  # revealed: None
+elif x is True:
+    reveal_type(x)  # revealed: Literal[True]
+else:
+    reveal_type(x)  # revealed: Literal[1]
 ```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is_not.md

@@ -5,10 +5,16 @@
 The type guard removes `None` from the union type:
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = None if flag else 1
 
 if x is not None:
     reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: None
 
 reveal_type(x)  # revealed: None | Literal[1]
 ```
@@ -16,18 +22,23 @@ reveal_type(x)  # revealed: None | Literal[1]
 ## `is not` for other singleton types
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = True if flag else False
 reveal_type(x)  # revealed: bool
 
 if x is not False:
     reveal_type(x)  # revealed: Literal[True]
+else:
+    reveal_type(x)  # revealed: Literal[False]
 ```
 
 ## `is not` for non-singleton types
 
-Non-singleton types should *not* narrow the type: two instances of a
-non-singleton class may occupy different addresses in memory even if
-they compare equal.
+Non-singleton types should *not* narrow the type: two instances of a non-singleton class may occupy
+different addresses in memory even if they compare equal.
 
 ```py
 x = 345
@@ -35,6 +46,27 @@ y = 345
 
 if x is not y:
     reveal_type(x)  # revealed: Literal[345]
+else:
+    reveal_type(x)  # revealed: Literal[345]
+```
+
+## `is not` for other types
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x = A()
+y = x if bool_instance() else None
+
+if y is not x:
+    reveal_type(y)  # revealed: A | None
+else:
+    reveal_type(y)  # revealed: A
+
+reveal_type(y)  # revealed: A | None
 ```
 
 ## `is not` in chained comparisons
@@ -42,6 +74,10 @@ if x is not y:
 The type guard removes `False` from the union type of the tested value only.
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+x_flag, y_flag = bool_instance(), bool_instance()
 x = True if x_flag else False
 y = True if y_flag else False
 
@@ -51,4 +87,10 @@ reveal_type(y)  # revealed: bool
 if y is not x is not False:  # Interpreted as `(y is not x) and (x is not False)`
     reveal_type(x)  # revealed: Literal[True]
     reveal_type(y)  # revealed: bool
+else:
+    # The negation of the clause above is (y is x) or (x is False)
+    # So we can't narrow the type of x or y here, because each arm of the `or` could be true
+
+    reveal_type(x)  # revealed: bool
+    reveal_type(y)  # revealed: bool
 ```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md

@@ -3,7 +3,8 @@
 ## Multiple negative contributions
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 x = int_instance()
 
@@ -16,6 +17,10 @@ if x != 1:
 ## Multiple negative contributions with simplification
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag1, flag2 = bool_instance(), bool_instance()
 x = 1 if flag1 else 2 if flag2 else 3
 
 if x != 1:
@@ -23,3 +28,29 @@ if x != 1:
     if x != 2:
         reveal_type(x)  # revealed: Literal[3]
 ```
+
+## elif-else blocks
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x != 1:
+    reveal_type(x)  # revealed: Literal[2, 3]
+    if x == 2:
+        # TODO should be `Literal[2]`
+        reveal_type(x)  # revealed: Literal[2, 3]
+    elif x == 3:
+        reveal_type(x)  # revealed: Literal[3]
+    else:
+        reveal_type(x)  # revealed: Never
+
+elif x != 2:
+    # TODO should be Literal[1]
+    reveal_type(x)  # revealed: Literal[1, 3]
+else:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not.md

@@ -0,0 +1,33 @@
+# Narrowing for `not` conditionals
+
+The `not` operator negates a constraint.
+
+## `not is None`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = None if bool_instance() else 1
+
+if not x is None:
+    reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: None
+
+reveal_type(x)  # revealed: None | Literal[1]
+```
+
+## `not isinstance`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else "a"
+
+if not isinstance(x, (int)):
+    reveal_type(x)  # revealed: Literal["a"]
+else:
+    reveal_type(x)  # revealed: Literal[1]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not_eq.md

@@ -3,24 +3,42 @@
 ## `x != None`
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = None if flag else 1
 
 if x != None:
     reveal_type(x)  # revealed: Literal[1]
+else:
+    # TODO should be None
+    reveal_type(x)  # revealed: None | Literal[1]
 ```
 
 ## `!=` for other singleton types
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = True if flag else False
 
 if x != False:
     reveal_type(x)  # revealed: Literal[True]
+else:
+    # TODO should be Literal[False]
+    reveal_type(x)  # revealed: bool
 ```
 
 ## `x != y` where `y` is of literal type
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else 2
 
 if x != 1:
@@ -30,6 +48,11 @@ if x != 1:
 ## `x != y` where `y` is a single-valued type
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 class A: ...
 class B: ...
 
@@ -37,6 +60,25 @@ C = A if flag else B
 
 if C != A:
     reveal_type(C)  # revealed: Literal[B]
+else:
+    # TODO should be Literal[A]
+    reveal_type(C)  # revealed: Literal[A, B]
+```
+
+## `x != y` where `y` has multiple single-valued options
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2
+y = 2 if bool_instance() else 3
+
+if x != y:
+    reveal_type(x)  # revealed: Literal[1, 2]
+else:
+    # TODO should be Literal[2]
+    reveal_type(x)  # revealed: Literal[1, 2]
 ```
 
 ## `!=` for non-single-valued types
@@ -44,11 +86,34 @@ if C != A:
 Only single-valued types should narrow the type:
 
 ```py
-def int_instance() -> int: ...
+def bool_instance() -> bool:
+    return True
 
+def int_instance() -> int:
+    return 42
+
+flag = bool_instance()
 x = int_instance() if flag else None
 y = int_instance()
 
 if x != y:
     reveal_type(x)  # revealed: int | None
 ```
+
+## Mix of single-valued and non-single-valued types
+
+```py
+def int_instance() -> int:
+    return 42
+
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2
+y = 2 if bool_instance() else int_instance()
+
+if x != y:
+    reveal_type(x)  # revealed: Literal[1, 2]
+else:
+    reveal_type(x)  # revealed: Literal[1, 2]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md

@@ -5,6 +5,11 @@ Narrowing for `isinstance(object, classinfo)` expressions.
 ## `classinfo` is a single type
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 x = 1 if flag else "a"
 
 if isinstance(x, int):
@@ -21,15 +26,21 @@ if isinstance(x, (int, object)):
 
 ## `classinfo` is a tuple of types
 
-Note: `isinstance(x, (int, str))` should not be confused with
-`isinstance(x, tuple[(int, str)])`. The former is equivalent to
-`isinstance(x, int | str)`:
+Note: `isinstance(x, (int, str))` should not be confused with `isinstance(x, tuple[(int, str)])`.
+The former is equivalent to `isinstance(x, int | str)`:
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag, flag1, flag2 = bool_instance(), bool_instance(), bool_instance()
+
 x = 1 if flag else "a"
 
 if isinstance(x, (int, str)):
     reveal_type(x)  # revealed: Literal[1] | Literal["a"]
+else:
+    reveal_type(x)  # revealed: Never
 
 if isinstance(x, (int, bytes)):
     reveal_type(x)  # revealed: Literal[1]
@@ -41,6 +52,8 @@ if isinstance(x, (bytes, str)):
 # one of the possibilities:
 if isinstance(x, (int, object)):
     reveal_type(x)  # revealed: Literal[1] | Literal["a"]
+else:
+    reveal_type(x)  # revealed: Never
 
 y = 1 if flag1 else "a" if flag2 else b"b"
 if isinstance(y, (int, str)):
@@ -56,10 +69,17 @@ if isinstance(y, (str, bytes)):
 ## `classinfo` is a nested tuple of types
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 x = 1 if flag else "a"
 
 if isinstance(x, (bool, (bytes, int))):
     reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: Literal["a"]
 ```
 
 ## Class types
@@ -67,6 +87,7 @@ if isinstance(x, (bool, (bytes, int))):
 ```py
 class A: ...
 class B: ...
+class C: ...
 
 def get_object() -> object: ...
 
@@ -76,11 +97,26 @@ if isinstance(x, A):
     reveal_type(x)  # revealed: A
     if isinstance(x, B):
         reveal_type(x)  # revealed: A & B
+    else:
+        reveal_type(x)  # revealed: A & ~B
+
+if isinstance(x, (A, B)):
+    reveal_type(x)  # revealed: A | B
+elif isinstance(x, (A, C)):
+    reveal_type(x)  # revealed: C & ~A & ~B
+else:
+    # TODO: Should be simplified to ~A & ~B & ~C
+    reveal_type(x)  # revealed: object & ~A & ~B & ~C
 ```
 
 ## No narrowing for instances of `builtins.type`
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 t = type("t", (), {})
 
 # This isn't testing what we want it to test if we infer anything more precise here:
@@ -94,6 +130,11 @@ if isinstance(x, t):
 ## Do not use custom `isinstance` for narrowing
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 def isinstance(x, t):
     return True
 
@@ -105,6 +146,11 @@ if isinstance(x, int):
 ## Do support narrowing if `isinstance` is aliased
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 isinstance_alias = isinstance
 
 x = 1 if flag else "a"
@@ -117,6 +163,10 @@ if isinstance_alias(x, int):
 ```py
 from builtins import isinstance as imported_isinstance
 
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else "a"
 if imported_isinstance(x, int):
     reveal_type(x)  # revealed: Literal[1]
@@ -125,6 +175,10 @@ if imported_isinstance(x, int):
 ## Do not narrow if second argument is not a type
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else "a"
 
 # TODO: this should cause us to emit a diagnostic during
@@ -141,6 +195,10 @@ if isinstance(x, "int"):
 ## Do not narrow if there are keyword arguments
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
 x = 1 if flag else "a"
 
 # TODO: this should cause us to emit a diagnostic

crates/red_knot_python_semantic/resources/mdtest/narrow/issubclass.md

@@ -0,0 +1,244 @@
+# Narrowing for `issubclass` checks
+
+Narrowing for `issubclass(class, classinfo)` expressions.
+
+## `classinfo` is a single type
+
+### Basic example
+
+```py
+def flag() -> bool: ...
+
+t = int if flag() else str
+
+if issubclass(t, bytes):
+    reveal_type(t)  # revealed: Never
+
+if issubclass(t, object):
+    reveal_type(t)  # revealed: Literal[int, str]
+
+if issubclass(t, int):
+    reveal_type(t)  # revealed: Literal[int]
+else:
+    reveal_type(t)  # revealed: Literal[str]
+
+if issubclass(t, str):
+    reveal_type(t)  # revealed: Literal[str]
+    if issubclass(t, int):
+        reveal_type(t)  # revealed: Never
+```
+
+### Proper narrowing in `elif` and `else` branches
+
+```py
+def flag() -> bool: ...
+
+t = int if flag() else str if flag() else bytes
+
+if issubclass(t, int):
+    reveal_type(t)  # revealed: Literal[int]
+else:
+    reveal_type(t)  # revealed: Literal[str, bytes]
+
+if issubclass(t, int):
+    reveal_type(t)  # revealed: Literal[int]
+elif issubclass(t, str):
+    reveal_type(t)  # revealed: Literal[str]
+else:
+    reveal_type(t)  # revealed: Literal[bytes]
+```
+
+### Multiple derived classes
+
+```py
+class Base: ...
+class Derived1(Base): ...
+class Derived2(Base): ...
+class Unrelated: ...
+
+def flag() -> bool: ...
+
+t1 = Derived1 if flag() else Derived2
+
+if issubclass(t1, Base):
+    reveal_type(t1)  # revealed: Literal[Derived1, Derived2]
+
+if issubclass(t1, Derived1):
+    reveal_type(t1)  # revealed: Literal[Derived1]
+else:
+    reveal_type(t1)  # revealed: Literal[Derived2]
+
+t2 = Derived1 if flag() else Base
+
+if issubclass(t2, Base):
+    reveal_type(t2)  # revealed: Literal[Derived1, Base]
+
+t3 = Derived1 if flag() else Unrelated
+
+if issubclass(t3, Base):
+    reveal_type(t3)  # revealed: Literal[Derived1]
+else:
+    reveal_type(t3)  # revealed: Literal[Unrelated]
+```
+
+### Narrowing for non-literals
+
+```py
+class A: ...
+class B: ...
+
+def get_class() -> type[object]: ...
+
+t = get_class()
+
+if issubclass(t, A):
+    reveal_type(t)  # revealed: type[A]
+    if issubclass(t, B):
+        reveal_type(t)  # revealed: type[A] & type[B]
+else:
+    reveal_type(t)  # revealed: type[object] & ~type[A]
+```
+
+### Handling of `None`
+
+```py
+from types import NoneType
+
+def flag() -> bool: ...
+
+t = int if flag() else NoneType
+
+if issubclass(t, NoneType):
+    reveal_type(t)  # revealed: Literal[NoneType]
+
+if issubclass(t, type(None)):
+    # TODO: this should be just `Literal[NoneType]`
+    reveal_type(t)  # revealed: Literal[int, NoneType]
+```
+
+## `classinfo` contains multiple types
+
+### (Nested) tuples of types
+
+```py
+class Unrelated: ...
+
+def flag() -> bool: ...
+
+t = int if flag() else str if flag() else bytes
+
+if issubclass(t, (int, (Unrelated, (bytes,)))):
+    reveal_type(t)  # revealed: Literal[int, bytes]
+else:
+    reveal_type(t)  # revealed: Literal[str]
+```
+
+## Special cases
+
+### Emit a diagnostic if the first argument is of wrong type
+
+#### Too wide
+
+`type[object]` is a subtype of `object`, but not every `object` can be passed as the first argument
+to `issubclass`:
+
+```py
+class A: ...
+
+def get_object() -> object: ...
+
+t = get_object()
+
+# TODO: we should emit a diagnostic here
+if issubclass(t, A):
+    reveal_type(t)  # revealed: type[A]
+```
+
+#### Wrong
+
+`Literal[1]` and `type` are entirely disjoint, so the inferred type of `Literal[1] & type[int]` is
+eagerly simplified to `Never` as a result of the type narrowing in the `if issubclass(t, int)`
+branch:
+
+```py
+t = 1
+
+# TODO: we should emit a diagnostic here
+if issubclass(t, int):
+    reveal_type(t)  # revealed: Never
+```
+
+### Do not use custom `issubclass` for narrowing
+
+```py
+def issubclass(c, ci):
+    return True
+
+def flag() -> bool: ...
+
+t = int if flag() else str
+if issubclass(t, int):
+    reveal_type(t)  # revealed: Literal[int, str]
+```
+
+### Do support narrowing if `issubclass` is aliased
+
+```py
+issubclass_alias = issubclass
+
+def flag() -> bool: ...
+
+t = int if flag() else str
+if issubclass_alias(t, int):
+    reveal_type(t)  # revealed: Literal[int]
+```
+
+### Do support narrowing if `issubclass` is imported
+
+```py
+from builtins import issubclass as imported_issubclass
+
+def flag() -> bool: ...
+
+t = int if flag() else str
+if imported_issubclass(t, int):
+    reveal_type(t)  # revealed: Literal[int]
+```
+
+### Do not narrow if second argument is not a proper `classinfo` argument
+
+```py
+from typing import Any
+
+def flag() -> bool: ...
+
+t = int if flag() else str
+
+# TODO: this should cause us to emit a diagnostic during
+# type checking
+if issubclass(t, "str"):
+    reveal_type(t)  # revealed: Literal[int, str]
+
+# TODO: this should cause us to emit a diagnostic during
+# type checking
+if issubclass(t, (bytes, "str")):
+    reveal_type(t)  # revealed: Literal[int, str]
+
+# TODO: this should cause us to emit a diagnostic during
+# type checking
+if issubclass(t, Any):
+    reveal_type(t)  # revealed: Literal[int, str]
+```
+
+### Do not narrow if there are keyword arguments
+
+```py
+def flag() -> bool: ...
+
+t = int if flag() else str
+
+# TODO: this should cause us to emit a diagnostic
+# (`issubclass` has no `foo` parameter)
+if issubclass(t, int, foo="bar"):
+    reveal_type(t)  # revealed: Literal[int, str]
+```

crates/red_knot_python_semantic/resources/mdtest/narrow/match.md

@@ -3,6 +3,11 @@
 ## Single `match` pattern
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 x = None if flag else 1
 reveal_type(x)  # revealed: None | Literal[1]
 

crates/red_knot_python_semantic/resources/mdtest/scopes/moduletype_attrs.md

@@ -0,0 +1,136 @@
+# Implicit globals from `types.ModuleType`
+
+## Implicit `ModuleType` globals
+
+All modules are instances of `types.ModuleType`. If a name can't be found in any local or global
+scope, we look it up as an attribute on `types.ModuleType` in typeshed before deciding that the name
+is unbound.
+
+```py
+reveal_type(__name__)  # revealed: str
+reveal_type(__file__)  # revealed: str | None
+reveal_type(__loader__)  # revealed: LoaderProtocol | None
+reveal_type(__package__)  # revealed: str | None
+reveal_type(__doc__)  # revealed: str | None
+
+# TODO: Should be `ModuleSpec | None`
+# (needs support for `*` imports)
+reveal_type(__spec__)  # revealed: Unknown | None
+
+# TODO: generics
+reveal_type(__path__)  # revealed: @Todo
+
+class X:
+    reveal_type(__name__)  # revealed: str
+
+def foo():
+    reveal_type(__name__)  # revealed: str
+```
+
+However, three attributes on `types.ModuleType` are not present as implicit module globals; these
+are excluded:
+
+```py path=unbound_dunders.py
+# error: [unresolved-reference]
+# revealed: Unknown
+reveal_type(__getattr__)
+
+# error: [unresolved-reference]
+# revealed: Unknown
+reveal_type(__dict__)
+
+# error: [unresolved-reference]
+# revealed: Unknown
+reveal_type(__init__)
+```
+
+## Accessed as attributes
+
+`ModuleType` attributes can also be accessed as attributes on module-literal types. The special
+attributes `__dict__` and `__init__`, and all attributes on `builtins.object`, can also be accessed
+as attributes on module-literal types, despite the fact that these are inaccessible as globals from
+inside the module:
+
+```py
+import typing
+
+reveal_type(typing.__name__)  # revealed: str
+reveal_type(typing.__init__)  # revealed: Literal[__init__]
+
+# These come from `builtins.object`, not `types.ModuleType`:
+reveal_type(typing.__eq__)  # revealed: Literal[__eq__]
+
+reveal_type(typing.__class__)  # revealed: Literal[type]
+
+# TODO: needs support for attribute access on instances, properties and generics;
+# should be `dict[str, Any]`
+reveal_type(typing.__dict__)  # revealed: @Todo
+```
+
+Typeshed includes a fake `__getattr__` method in the stub for `types.ModuleType` to help out with
+dynamic imports; but we ignore that for module-literal types where we know exactly which module
+we're dealing with:
+
+```py path=__getattr__.py
+import typing
+
+reveal_type(typing.__getattr__)  # revealed: Unknown
+```
+
+## `types.ModuleType.__dict__` takes precedence over global variable `__dict__`
+
+It's impossible to override the `__dict__` attribute of `types.ModuleType` instances from inside the
+module; we should prioritise the attribute in the `types.ModuleType` stub over a variable named
+`__dict__` in the module's global namespace:
+
+```py path=foo.py
+__dict__ = "foo"
+
+reveal_type(__dict__)  # revealed: Literal["foo"]
+```
+
+```py path=bar.py
+import foo
+from foo import __dict__ as foo_dict
+
+# TODO: needs support for attribute access on instances, properties, and generics;
+# should be `dict[str, Any]` for both of these:
+reveal_type(foo.__dict__)  # revealed: @Todo
+reveal_type(foo_dict)  # revealed: @Todo
+```
+
+## Conditionally global or `ModuleType` attribute
+
+Attributes overridden in the module namespace take priority. If a builtin name is conditionally
+defined as a global, however, a name lookup should union the `ModuleType` type with the
+conditionally defined type:
+
+```py
+__file__ = 42
+
+def returns_bool() -> bool:
+    return True
+
+if returns_bool():
+    __name__ = 1
+
+reveal_type(__file__)  # revealed: Literal[42]
+reveal_type(__name__)  # revealed: Literal[1] | str
+```
+
+## Conditionally global or `ModuleType` attribute, with annotation
+
+The same is true if the name is annotated:
+
+```py
+__file__: int = 42
+
+def returns_bool() -> bool:
+    return True
+
+if returns_bool():
+    __name__: int = 1
+
+reveal_type(__file__)  # revealed: Literal[42]
+reveal_type(__name__)  # revealed: Literal[1] | str
+```

crates/red_knot_python_semantic/resources/mdtest/shadowing/function.md

@@ -2,7 +2,8 @@
 
 ## Parameter
 
-Parameter `x` of type `str` is shadowed and reassigned with a new `int` value inside the function. No diagnostics should be generated.
+Parameter `x` of type `str` is shadowed and reassigned with a new `int` value inside the function.
+No diagnostics should be generated.
 
 ```py path=a.py
 def f(x: str):

crates/red_knot_python_semantic/resources/mdtest/shadowing/variable_declaration.md

@@ -3,6 +3,11 @@
 ## Shadow after incompatible declarations is OK
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     x: str
 else:

crates/red_knot_python_semantic/resources/mdtest/stubs/class.md

@@ -2,10 +2,16 @@
 
 ## Cyclical class definition
 
-In type stubs, classes can reference themselves in their base class definitions. For example, in `typeshed`, we have `class str(Sequence[str]): ...`.
+In type stubs, classes can reference themselves in their base class definitions. For example, in
+`typeshed`, we have `class str(Sequence[str]): ...`.
 
 ```py path=a.pyi
-class C(C): ...
+class Foo[T]: ...
 
-reveal_type(C)  # revealed: Literal[C]
+# TODO: actually is subscriptable
+# error: [non-subscriptable]
+class Bar(Foo[Bar]): ...
+
+reveal_type(Bar)  # revealed: Literal[Bar]
+reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
 ```

crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md

@@ -1,6 +1,6 @@
-# Bytes subscript
+# Bytes subscripts
 
-## Simple
+## Indexing
 
 ```py
 b = b"\x00abc\xff"
@@ -21,14 +21,37 @@ reveal_type(x)  # revealed: Unknown
 
 y = b[-6]  # error: [index-out-of-bounds] "Index -6 is out of bounds for bytes literal `Literal[b"\x00abc\xff"]` with length 5"
 reveal_type(y)  # revealed: Unknown
-```
 
-## Function return
-
-```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = b"abcde"[int_instance()]
 # TODO: Support overloads... Should be `bytes`
 reveal_type(a)  # revealed: @Todo
 ```
+
+## Slices
+
+```py
+b = b"\x00abc\xff"
+
+reveal_type(b[0:2])  # revealed: Literal[b"\x00a"]
+reveal_type(b[-3:])  # revealed: Literal[b"bc\xff"]
+
+b[0:4:0]  # error: [zero-stepsize-in-slice]
+b[:4:0]  # error: [zero-stepsize-in-slice]
+b[0::0]  # error: [zero-stepsize-in-slice]
+b[::0]  # error: [zero-stepsize-in-slice]
+
+def int_instance() -> int: ...
+
+byte_slice1 = b[int_instance() : int_instance()]
+# TODO: Support overloads... Should be `bytes`
+reveal_type(byte_slice1)  # revealed: @Todo
+
+def bytes_instance() -> bytes: ...
+
+byte_slice2 = bytes_instance()[0:5]
+# TODO: Support overloads... Should be `bytes`
+reveal_type(byte_slice2)  # revealed: @Todo
+```

crates/red_knot_python_semantic/resources/mdtest/subscript/class.md

@@ -39,7 +39,8 @@ reveal_type(UnionClassGetItem[0])  # revealed: str | int
 ## Class getitem with class union
 
 ```py
-flag = True
+def bool_instance() -> bool:
+    return True
 
 class A:
     def __class_getitem__(cls, item: int) -> str:
@@ -49,7 +50,7 @@ class B:
     def __class_getitem__(cls, item: int) -> int:
         return item
 
-x = A if flag else B
+x = A if bool_instance() else B
 
 reveal_type(x)  # revealed: Literal[A, B]
 reveal_type(x[0])  # revealed: str | int
@@ -68,8 +69,8 @@ if flag:
 else:
     class Spam: ...
 
-# error: [call-non-callable] "Method `__class_getitem__` of type `Literal[__class_getitem__] | Unbound` is not callable on object of type `Literal[Spam, Spam]`"
-# revealed: str | Unknown
+# error: [call-possibly-unbound-method] "Method `__class_getitem__` of type `Literal[Spam, Spam]` is possibly unbound"
+# revealed: str
 reveal_type(Spam[42])
 ```
 

crates/red_knot_python_semantic/resources/mdtest/subscript/lists.md

@@ -23,8 +23,7 @@ reveal_type(x["a"])  # revealed: @Todo
 
 ## Assignments within list assignment
 
-In assignment, we might also have a named assignment.
-This should also get type checked.
+In assignment, we might also have a named assignment. This should also get type checked.
 
 ```py
 x = [1, 2, 3]

crates/red_knot_python_semantic/resources/mdtest/subscript/stepsize_zero.md

@@ -0,0 +1,13 @@
+# Stepsize zero in slices
+
+We raise a `zero-stepsize-in-slice` diagnostic when trying to slice a literal string, bytes, or
+tuple with a step size of zero (see tests in `string.md`, `bytes.md` and `tuple.md`). But we don't
+want to raise this diagnostic when slicing a custom type:
+
+```py
+class MySequence:
+    def __getitem__(self, s: slice) -> int:
+        return 0
+
+MySequence()[0:1:0]  # No error
+```

crates/red_knot_python_semantic/resources/mdtest/subscript/string.md

@@ -1,6 +1,6 @@
-# Subscript on strings
+# String subscripts
 
-## Simple
+## Indexing
 
 ```py
 s = "abcde"
@@ -18,14 +18,82 @@ reveal_type(a)  # revealed: Unknown
 
 b = s[-8]  # error: [index-out-of-bounds] "Index -8 is out of bounds for string `Literal["abcde"]` with length 5"
 reveal_type(b)  # revealed: Unknown
-```
 
-## Function return
-
-```py
 def int_instance() -> int: ...
 
 a = "abcde"[int_instance()]
 # TODO: Support overloads... Should be `str`
 reveal_type(a)  # revealed: @Todo
 ```
+
+## Slices
+
+```py
+s = "abcde"
+
+reveal_type(s[0:0])  # revealed: Literal[""]
+reveal_type(s[0:1])  # revealed: Literal["a"]
+reveal_type(s[0:2])  # revealed: Literal["ab"]
+reveal_type(s[0:5])  # revealed: Literal["abcde"]
+reveal_type(s[0:6])  # revealed: Literal["abcde"]
+reveal_type(s[1:3])  # revealed: Literal["bc"]
+
+reveal_type(s[-3:5])  # revealed: Literal["cde"]
+reveal_type(s[-4:-2])  # revealed: Literal["bc"]
+reveal_type(s[-10:10])  # revealed: Literal["abcde"]
+
+reveal_type(s[0:])  # revealed: Literal["abcde"]
+reveal_type(s[2:])  # revealed: Literal["cde"]
+reveal_type(s[5:])  # revealed: Literal[""]
+reveal_type(s[:2])  # revealed: Literal["ab"]
+reveal_type(s[:0])  # revealed: Literal[""]
+reveal_type(s[:2])  # revealed: Literal["ab"]
+reveal_type(s[:10])  # revealed: Literal["abcde"]
+reveal_type(s[:])  # revealed: Literal["abcde"]
+
+reveal_type(s[::-1])  # revealed: Literal["edcba"]
+reveal_type(s[::2])  # revealed: Literal["ace"]
+reveal_type(s[-2:-5:-1])  # revealed: Literal["dcb"]
+reveal_type(s[::-2])  # revealed: Literal["eca"]
+reveal_type(s[-1::-3])  # revealed: Literal["eb"]
+
+reveal_type(s[None:2:None])  # revealed: Literal["ab"]
+reveal_type(s[1:None:1])  # revealed: Literal["bcde"]
+reveal_type(s[None:None:None])  # revealed: Literal["abcde"]
+
+start = 1
+stop = None
+step = 2
+reveal_type(s[start:stop:step])  # revealed: Literal["bd"]
+
+reveal_type(s[False:True])  # revealed: Literal["a"]
+reveal_type(s[True:3])  # revealed: Literal["bc"]
+
+s[0:4:0]  # error: [zero-stepsize-in-slice]
+s[:4:0]  # error: [zero-stepsize-in-slice]
+s[0::0]  # error: [zero-stepsize-in-slice]
+s[::0]  # error: [zero-stepsize-in-slice]
+
+def int_instance() -> int: ...
+
+substring1 = s[int_instance() : int_instance()]
+# TODO: Support overloads... Should be `LiteralString`
+reveal_type(substring1)  # revealed: @Todo
+
+def str_instance() -> str: ...
+
+substring2 = str_instance()[0:5]
+# TODO: Support overloads... Should be `str`
+reveal_type(substring2)  # revealed: @Todo
+```
+
+## Unsupported slice types
+
+```py
+# TODO: It would be great if we raised an error here. This can be done once
+# we have support for overloads and generics, and once typeshed has a more
+# precise annotation for `str.__getitem__`, that makes use of the generic
+# `slice[..]` type. We could then infer `slice[str, str]` here and see that
+# it doesn't match the signature of `str.__getitem__`.
+"foo"["bar":"baz"]
+```

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -1,6 +1,6 @@
 # Tuple subscripts
 
-## Basic
+## Indexing
 
 ```py
 t = (1, "a", "b")
@@ -10,9 +10,66 @@ reveal_type(t[1])  # revealed: Literal["a"]
 reveal_type(t[-1])  # revealed: Literal["b"]
 reveal_type(t[-2])  # revealed: Literal["a"]
 
+reveal_type(t[False])  # revealed: Literal[1]
+reveal_type(t[True])  # revealed: Literal["a"]
+
 a = t[4]  # error: [index-out-of-bounds]
 reveal_type(a)  # revealed: Unknown
 
 b = t[-4]  # error: [index-out-of-bounds]
 reveal_type(b)  # revealed: Unknown
 ```
+
+## Slices
+
+```py
+t = (1, "a", None, b"b")
+
+reveal_type(t[0:0])  # revealed: tuple[()]
+reveal_type(t[0:1])  # revealed: tuple[Literal[1]]
+reveal_type(t[0:2])  # revealed: tuple[Literal[1], Literal["a"]]
+reveal_type(t[0:4])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+reveal_type(t[0:5])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+reveal_type(t[1:3])  # revealed: tuple[Literal["a"], None]
+
+reveal_type(t[-2:4])  # revealed: tuple[None, Literal[b"b"]]
+reveal_type(t[-3:-1])  # revealed: tuple[Literal["a"], None]
+reveal_type(t[-10:10])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+
+reveal_type(t[0:])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+reveal_type(t[2:])  # revealed: tuple[None, Literal[b"b"]]
+reveal_type(t[4:])  # revealed: tuple[()]
+reveal_type(t[:0])  # revealed: tuple[()]
+reveal_type(t[:2])  # revealed: tuple[Literal[1], Literal["a"]]
+reveal_type(t[:10])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+reveal_type(t[:])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+
+reveal_type(t[::-1])  # revealed: tuple[Literal[b"b"], None, Literal["a"], Literal[1]]
+reveal_type(t[::2])  # revealed: tuple[Literal[1], None]
+reveal_type(t[-2:-5:-1])  # revealed: tuple[None, Literal["a"], Literal[1]]
+reveal_type(t[::-2])  # revealed: tuple[Literal[b"b"], Literal["a"]]
+reveal_type(t[-1::-3])  # revealed: tuple[Literal[b"b"], Literal[1]]
+
+reveal_type(t[None:2:None])  # revealed: tuple[Literal[1], Literal["a"]]
+reveal_type(t[1:None:1])  # revealed: tuple[Literal["a"], None, Literal[b"b"]]
+reveal_type(t[None:None:None])  # revealed: tuple[Literal[1], Literal["a"], None, Literal[b"b"]]
+
+start = 1
+stop = None
+step = 2
+reveal_type(t[start:stop:step])  # revealed: tuple[Literal["a"], Literal[b"b"]]
+
+reveal_type(t[False:True])  # revealed: tuple[Literal[1]]
+reveal_type(t[True:3])  # revealed: tuple[Literal["a"], None]
+
+t[0:4:0]  # error: [zero-stepsize-in-slice]
+t[:4:0]  # error: [zero-stepsize-in-slice]
+t[0::0]  # error: [zero-stepsize-in-slice]
+t[::0]  # error: [zero-stepsize-in-slice]
+
+def int_instance() -> int: ...
+
+tuple_slice = t[int_instance() : int_instance()]
+# TODO: Support overloads... Should be `tuple[Literal[1, 'a', b"b"] | None, ...]`
+reveal_type(tuple_slice)  # revealed: @Todo
+```

crates/red_knot_python_semantic/resources/mdtest/unary/instance.md

@@ -1,6 +1,8 @@
 # Unary Operations
 
 ```py
+from typing import Literal
+
 class Number:
     def __init__(self, value: int):
         self.value = 1
@@ -18,7 +20,7 @@ a = Number()
 
 reveal_type(+a)  # revealed: int
 reveal_type(-a)  # revealed: int
-reveal_type(~a)  # revealed: @Todo
+reveal_type(~a)  # revealed: Literal[True]
 
 class NoDunder: ...
 

crates/red_knot_python_semantic/resources/mdtest/unary/not.md

@@ -37,6 +37,11 @@ y = 1
 ## Union
 
 ```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
 if flag:
     p = 1
     q = 3.3

crates/red_knot_python_semantic/resources/mdtest/unpacking.md

@@ -81,8 +81,7 @@ reveal_type(b)  # revealed: Literal[2]
 
 ```py
 # TODO: Add diagnostic (need more values to unpack)
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c, d] = (1, 2)  # error: "Object of type `None` is not iterable"
+[a, *b, c, d] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -93,7 +92,7 @@ reveal_type(d)  # revealed: Unknown
 ### Starred expression (2)
 
 ```py
-[a, *b, c] = (1, 2)  # error: "Object of type `None` is not iterable"
+[a, *b, c] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -103,8 +102,7 @@ reveal_type(c)  # revealed: Literal[2]
 ### Starred expression (3)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c] = (1, 2, 3)  # error: "Object of type `None` is not iterable"
+[a, *b, c] = (1, 2, 3)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -114,8 +112,7 @@ reveal_type(c)  # revealed: Literal[3]
 ### Starred expression (4)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c, d] = (1, 2, 3, 4, 5, 6)  # error: "Object of type `None` is not iterable"
+[a, *b, c, d] = (1, 2, 3, 4, 5, 6)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -126,22 +123,29 @@ reveal_type(d)  # revealed: Literal[6]
 ### Starred expression (5)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, b, *c] = (1, 2, 3, 4)  # error: "Object of type `None` is not iterable"
+[a, b, *c] = (1, 2, 3, 4)
 reveal_type(a)  # revealed: Literal[1]
 reveal_type(b)  # revealed: Literal[2]
 # TODO: Should be list[int] once support for assigning to starred expression is added
 reveal_type(c)  # revealed: @Todo
 ```
 
-### Non-iterable unpacking
-
-TODO: Remove duplicate diagnostics. This is happening because for a sequence-like
-assignment target, multiple definitions are created and the inference engine runs
-on each of them which results in duplicate diagnostics.
+### Starred expression (6)
+
+```py
+# TODO: Add diagnostic (need more values to unpack)
+(a, b, c, *d, e, f) = (1,)
+reveal_type(a)  # revealed: Literal[1]
+reveal_type(b)  # revealed: Unknown
+reveal_type(c)  # revealed: Unknown
+reveal_type(d)  # revealed: @Todo
+reveal_type(e)  # revealed: Unknown
+reveal_type(f)  # revealed: Unknown
+```
+
+### Non-iterable unpacking
 
 ```py
-# error: "Object of type `Literal[1]` is not iterable"
 # error: "Object of type `Literal[1]` is not iterable"
 a, b = 1
 reveal_type(a)  # revealed: Unknown
@@ -215,8 +219,7 @@ reveal_type(b)  # revealed: LiteralString
 
 ```py
 # TODO: Add diagnostic (need more values to unpack)
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c, d) = "ab"  # error: "Object of type `None` is not iterable"
+(a, *b, c, d) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -227,7 +230,7 @@ reveal_type(d)  # revealed: Unknown
 ### Starred expression (2)
 
 ```py
-(a, *b, c) = "ab"  # error: "Object of type `None` is not iterable"
+(a, *b, c) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -237,8 +240,7 @@ reveal_type(c)  # revealed: LiteralString
 ### Starred expression (3)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c) = "abc"  # error: "Object of type `None` is not iterable"
+(a, *b, c) = "abc"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -248,8 +250,7 @@ reveal_type(c)  # revealed: LiteralString
 ### Starred expression (4)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c, d) = "abcdef"  # error: "Object of type `None` is not iterable"
+(a, *b, c, d) = "abcdef"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -260,8 +261,7 @@ reveal_type(d)  # revealed: LiteralString
 ### Starred expression (5)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, b, *c) = "abcd"  # error: "Object of type `None` is not iterable"
+(a, b, *c) = "abcd"
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: LiteralString
 # TODO: Should be list[int] once support for assigning to starred expression is added

crates/red_knot_python_semantic/resources/mdtest/with/async_with.md

@@ -0,0 +1,21 @@
+# Async with statements
+
+## Basic `async with` statement
+
+The type of the target variable in a `with` statement should be the return type from the context
+manager's `__aenter__` method. However, `async with` statements aren't supported yet. This test
+asserts that it doesn't emit any context manager-related errors.
+
+```py
+class Target: ...
+
+class Manager:
+    async def __aenter__(self) -> Target:
+        return Target()
+
+    async def __aexit__(self, exc_type, exc_value, traceback): ...
+
+async def test():
+    async with Manager() as f:
+        reveal_type(f)  # revealed: @Todo
+```

crates/red_knot_python_semantic/resources/mdtest/with/with.md

@@ -0,0 +1,141 @@
+# With statements
+
+## Basic `with` statement
+
+The type of the target variable in a `with` statement is the return type from the context manager's
+`__enter__` method.
+
+```py
+class Target: ...
+
+class Manager:
+    def __enter__(self) -> Target:
+        return Target()
+
+    def __exit__(self, exc_type, exc_value, traceback): ...
+
+with Manager() as f:
+    reveal_type(f)  # revealed: Target
+```
+
+## Union context manager
+
+```py
+def coinflip() -> bool:
+    return True
+
+class Manager1:
+    def __enter__(self) -> str:
+        return "foo"
+
+    def __exit__(self, exc_type, exc_value, traceback): ...
+
+class Manager2:
+    def __enter__(self) -> int:
+        return 42
+
+    def __exit__(self, exc_type, exc_value, traceback): ...
+
+context_expr = Manager1() if coinflip() else Manager2()
+
+with context_expr as f:
+    reveal_type(f)  # revealed: str | int
+```
+
+## Context manager without an `__enter__` or `__exit__` method
+
+```py
+class Manager: ...
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it doesn't implement `__enter__` and `__exit__`"
+with Manager():
+    ...
+```
+
+## Context manager without an `__enter__` method
+
+```py
+class Manager:
+    def __exit__(self, exc_tpe, exc_value, traceback): ...
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it doesn't implement `__enter__`"
+with Manager():
+    ...
+```
+
+## Context manager without an `__exit__` method
+
+```py
+class Manager:
+    def __enter__(self): ...
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because it doesn't implement `__exit__`"
+with Manager():
+    ...
+```
+
+## Context manager with non-callable `__enter__` attribute
+
+```py
+class Manager:
+    __enter__ = 42
+
+    def __exit__(self, exc_tpe, exc_value, traceback): ...
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because the method `__enter__` of type `Literal[42]` is not callable"
+with Manager():
+    ...
+```
+
+## Context manager with non-callable `__exit__` attribute
+
+```py
+class Manager:
+    def __enter__(self) -> Self: ...
+
+    __exit__ = 32
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because the method `__exit__` of type `Literal[32]` is not callable"
+with Manager():
+    ...
+```
+
+## Context expression with possibly-unbound union variants
+
+```py
+def coinflip() -> bool:
+    return True
+
+class Manager1:
+    def __enter__(self) -> str:
+        return "foo"
+
+    def __exit__(self, exc_type, exc_value, traceback): ...
+
+class NotAContextManager: ...
+
+context_expr = Manager1() if coinflip() else NotAContextManager()
+
+# error: [invalid-context-manager] "Object of type `Manager1 | NotAContextManager` cannot be used with `with` because the method `__enter__` is possibly unbound"
+# error: [invalid-context-manager] "Object of type `Manager1 | NotAContextManager` cannot be used with `with` because the method `__exit__` is possibly unbound"
+with context_expr as f:
+    reveal_type(f)  # revealed: str
+```
+
+## Context expression with "sometimes" callable `__enter__` method
+
+```py
+def coinflip() -> bool:
+    return True
+
+class Manager:
+    if coinflip():
+        def __enter__(self) -> str:
+            return "abcd"
+
+    def __exit__(self, *args): ...
+
+# error: [invalid-context-manager] "Object of type `Manager` cannot be used with `with` because the method `__enter__` is possibly unbound"
+with Manager() as f:
+    reveal_type(f)  # revealed: str
+```

crates/red_knot_python_semantic/src/ast_node_ref.rs

@@ -132,7 +132,7 @@ mod tests {
     #[test]
     fn inequality() {
         let parsed_raw = parse_unchecked_source("1 + 2", PySourceType::Python);
-        let parsed = ParsedModule::new(parsed_raw.clone());
+        let parsed = ParsedModule::new(parsed_raw);
 
         let stmt = &parsed.syntax().body[0];
         let node = unsafe { AstNodeRef::new(parsed.clone(), stmt) };
@@ -150,7 +150,7 @@ mod tests {
     #[allow(unsafe_code)]
     fn debug() {
         let parsed_raw = parse_unchecked_source("1 + 2", PySourceType::Python);
-        let parsed = ParsedModule::new(parsed_raw.clone());
+        let parsed = ParsedModule::new(parsed_raw);
 
         let stmt = &parsed.syntax().body[0];
 

crates/red_knot_python_semantic/src/lib.rs

@@ -20,7 +20,9 @@ pub mod semantic_index;
 mod semantic_model;
 pub(crate) mod site_packages;
 mod stdlib;
+pub(crate) mod symbol;
 pub mod types;
+mod unpack;
 mod util;
 
 type FxOrderSet<V> = ordermap::set::OrderSet<V, BuildHasherDefault<FxHasher>>;

crates/red_knot_python_semantic/src/module_resolver/resolver.rs

@@ -1294,7 +1294,7 @@ mod tests {
                 search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: src.clone(),
-                    custom_typeshed: Some(custom_typeshed.clone()),
+                    custom_typeshed: Some(custom_typeshed),
                     site_packages: SitePackages::Known(vec![site_packages]),
                 },
             },
@@ -1445,7 +1445,7 @@ mod tests {
         assert_function_query_was_not_run(
             &db,
             resolve_module_query,
-            ModuleNameIngredient::new(&db, functools_module_name.clone()),
+            ModuleNameIngredient::new(&db, functools_module_name),
             &events,
         );
         assert_eq!(functools_module.search_path(), &stdlib);

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -125,6 +125,7 @@ impl<'db> SemanticIndex<'db> {
     ///
     /// Use the Salsa cached [`symbol_table()`] query if you only need the
     /// symbol table for a single scope.
+    #[track_caller]
     pub(super) fn symbol_table(&self, scope_id: FileScopeId) -> Arc<SymbolTable> {
         self.symbol_tables[scope_id].clone()
     }
@@ -133,15 +134,18 @@ impl<'db> SemanticIndex<'db> {
     ///
     /// Use the Salsa cached [`use_def_map()`] query if you only need the
     /// use-def map for a single scope.
+    #[track_caller]
     pub(super) fn use_def_map(&self, scope_id: FileScopeId) -> Arc<UseDefMap> {
         self.use_def_maps[scope_id].clone()
     }
 
+    #[track_caller]
     pub(crate) fn ast_ids(&self, scope_id: FileScopeId) -> &AstIds {
         &self.ast_ids[scope_id]
     }
 
     /// Returns the ID of the `expression`'s enclosing scope.
+    #[track_caller]
     pub(crate) fn expression_scope_id(
         &self,
         expression: impl Into<ExpressionNodeKey>,
@@ -151,11 +155,13 @@ impl<'db> SemanticIndex<'db> {
 
     /// Returns the [`Scope`] of the `expression`'s enclosing scope.
     #[allow(unused)]
+    #[track_caller]
     pub(crate) fn expression_scope(&self, expression: impl Into<ExpressionNodeKey>) -> &Scope {
         &self.scopes[self.expression_scope_id(expression)]
     }
 
     /// Returns the [`Scope`] with the given id.
+    #[track_caller]
     pub(crate) fn scope(&self, id: FileScopeId) -> &Scope {
         &self.scopes[id]
     }
@@ -172,6 +178,7 @@ impl<'db> SemanticIndex<'db> {
 
     /// Returns the parent scope of `scope_id`.
     #[allow(unused)]
+    #[track_caller]
     pub(crate) fn parent_scope(&self, scope_id: FileScopeId) -> Option<&Scope> {
         Some(&self.scopes[self.parent_scope_id(scope_id)?])
     }
@@ -195,6 +202,7 @@ impl<'db> SemanticIndex<'db> {
     }
 
     /// Returns the [`Definition`] salsa ingredient for `definition_key`.
+    #[track_caller]
     pub(crate) fn definition(
         &self,
         definition_key: impl Into<DefinitionNodeKey>,
@@ -206,6 +214,7 @@ impl<'db> SemanticIndex<'db> {
     /// Panics if we have no expression ingredient for that node. We can only call this method for
     /// standalone-inferable expressions, which we call `add_standalone_expression` for in
     /// [`SemanticIndexBuilder`].
+    #[track_caller]
     pub(crate) fn expression(
         &self,
         expression_key: impl Into<ExpressionNodeKey>,
@@ -213,8 +222,18 @@ impl<'db> SemanticIndex<'db> {
         self.expressions_by_node[&expression_key.into()]
     }
 
+    pub(crate) fn try_expression(
+        &self,
+        expression_key: impl Into<ExpressionNodeKey>,
+    ) -> Option<Expression<'db>> {
+        self.expressions_by_node
+            .get(&expression_key.into())
+            .copied()
+    }
+
     /// Returns the id of the scope that `node` creates. This is different from [`Definition::scope`] which
     /// returns the scope in which that definition is defined in.
+    #[track_caller]
     pub(crate) fn node_scope(&self, node: NodeWithScopeRef) -> FileScopeId {
         self.scopes_by_node[&node.node_key()]
     }

crates/red_knot_python_semantic/src/semantic_index/ast_ids.rs

@@ -87,6 +87,14 @@ pub trait HasScopedAstId {
     fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id;
 }
 
+impl<T: HasScopedAstId> HasScopedAstId for Box<T> {
+    type Id = <T as HasScopedAstId>::Id;
+
+    fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
+        self.as_ref().scoped_ast_id(db, scope)
+    }
+}
+
 /// Uniquely identifies an [`ast::Expr`] in a [`crate::semantic_index::symbol::FileScopeId`].
 #[newtype_index]
 pub struct ScopedExpressionId;

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -9,7 +9,7 @@ use ruff_index::IndexVec;
 use ruff_python_ast as ast;
 use ruff_python_ast::name::Name;
 use ruff_python_ast::visitor::{walk_expr, walk_pattern, walk_stmt, Visitor};
-use ruff_python_ast::AnyParameterRef;
+use ruff_python_ast::{AnyParameterRef, BoolOp, Expr};
 
 use crate::ast_node_ref::AstNodeRef;
 use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
@@ -25,12 +25,13 @@ use crate::semantic_index::symbol::{
 };
 use crate::semantic_index::use_def::{FlowSnapshot, UseDefMapBuilder};
 use crate::semantic_index::SemanticIndex;
+use crate::unpack::Unpack;
 use crate::Db;
 
-use super::constraint::{Constraint, PatternConstraint};
+use super::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use super::definition::{
-    AssignmentKind, DefinitionCategory, ExceptHandlerDefinitionNodeRef,
-    MatchPatternDefinitionNodeRef, WithItemDefinitionNodeRef,
+    DefinitionCategory, ExceptHandlerDefinitionNodeRef, MatchPatternDefinitionNodeRef,
+    WithItemDefinitionNodeRef,
 };
 
 mod except_handlers;
@@ -46,6 +47,7 @@ pub(super) struct SemanticIndexBuilder<'db> {
     current_assignments: Vec<CurrentAssignment<'db>>,
     /// The match case we're currently visiting.
     current_match_case: Option<CurrentMatchCase<'db>>,
+
     /// Flow states at each `break` in the current loop.
     loop_break_states: Vec<FlowSnapshot>,
     /// Per-scope contexts regarding nested `try`/`except` statements
@@ -112,9 +114,11 @@ impl<'db> SemanticIndexBuilder<'db> {
     fn push_scope_with_parent(&mut self, node: NodeWithScopeRef, parent: Option<FileScopeId>) {
         let children_start = self.scopes.next_index() + 1;
 
+        #[allow(unsafe_code)]
         let scope = Scope {
             parent,
-            kind: node.scope_kind(),
+            // SAFETY: `node` is guaranteed to be a child of `self.module`
+            node: unsafe { node.to_kind(self.module.clone()) },
             descendents: children_start..children_start,
         };
         self.try_node_context_stack_manager.enter_nested_scope();
@@ -124,15 +128,7 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.use_def_maps.push(UseDefMapBuilder::new());
         let ast_id_scope = self.ast_ids.push(AstIdsBuilder::new());
 
-        #[allow(unsafe_code)]
-        // SAFETY: `node` is guaranteed to be a child of `self.module`
-        let scope_id = ScopeId::new(
-            self.db,
-            self.file,
-            file_scope_id,
-            unsafe { node.to_kind(self.module.clone()) },
-            countme::Count::default(),
-        );
+        let scope_id = ScopeId::new(self.db, self.file, file_scope_id, countme::Count::default());
 
         self.scope_ids_by_scope.push(scope_id);
         self.scopes_by_node.insert(node.node_key(), file_scope_id);
@@ -195,14 +191,18 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.current_symbol_table().mark_symbol_bound(id);
     }
 
+    fn mark_symbol_declared(&mut self, id: ScopedSymbolId) {
+        self.current_symbol_table().mark_symbol_declared(id);
+    }
+
     fn mark_symbol_used(&mut self, id: ScopedSymbolId) {
         self.current_symbol_table().mark_symbol_used(id);
     }
 
-    fn add_definition<'a>(
+    fn add_definition(
         &mut self,
         symbol: ScopedSymbolId,
-        definition_node: impl Into<DefinitionNodeRef<'a>>,
+        definition_node: impl Into<DefinitionNodeRef<'db>>,
     ) -> Definition<'db> {
         let definition_node: DefinitionNodeRef<'_> = definition_node.into();
         #[allow(unsafe_code)]
@@ -226,6 +226,9 @@ impl<'db> SemanticIndexBuilder<'db> {
         if category.is_binding() {
             self.mark_symbol_bound(symbol);
         }
+        if category.is_declaration() {
+            self.mark_symbol_declared(symbol);
+        }
 
         let use_def = self.current_use_def_map_mut();
         match category {
@@ -243,12 +246,30 @@ impl<'db> SemanticIndexBuilder<'db> {
         definition
     }
 
-    fn add_expression_constraint(&mut self, constraint_node: &ast::Expr) -> Expression<'db> {
-        let expression = self.add_standalone_expression(constraint_node);
-        self.current_use_def_map_mut()
-            .record_constraint(Constraint::Expression(expression));
+    fn record_expression_constraint(&mut self, constraint_node: &ast::Expr) -> Constraint<'db> {
+        let constraint = self.build_constraint(constraint_node);
+        self.record_constraint(constraint);
+        constraint
+    }
 
-        expression
+    fn record_constraint(&mut self, constraint: Constraint<'db>) {
+        self.current_use_def_map_mut().record_constraint(constraint);
+    }
+
+    fn build_constraint(&mut self, constraint_node: &Expr) -> Constraint<'db> {
+        let expression = self.add_standalone_expression(constraint_node);
+        Constraint {
+            node: ConstraintNode::Expression(expression),
+            is_positive: true,
+        }
+    }
+
+    fn record_negated_constraint(&mut self, constraint: Constraint<'db>) {
+        self.current_use_def_map_mut()
+            .record_constraint(Constraint {
+                node: constraint.node,
+                is_positive: false,
+            });
     }
 
     fn push_assignment(&mut self, assignment: CurrentAssignment<'db>) {
@@ -260,8 +281,12 @@ impl<'db> SemanticIndexBuilder<'db> {
         debug_assert!(popped_assignment.is_some());
     }
 
-    fn current_assignment(&self) -> Option<&CurrentAssignment<'db>> {
-        self.current_assignments.last()
+    fn current_assignment(&self) -> Option<CurrentAssignment<'db>> {
+        self.current_assignments.last().copied()
+    }
+
+    fn current_assignment_mut(&mut self) -> Option<&mut CurrentAssignment<'db>> {
+        self.current_assignments.last_mut()
     }
 
     fn add_pattern_constraint(
@@ -285,7 +310,10 @@ impl<'db> SemanticIndexBuilder<'db> {
             countme::Count::default(),
         );
         self.current_use_def_map_mut()
-            .record_constraint(Constraint::Pattern(pattern_constraint));
+            .record_constraint(Constraint {
+                node: ConstraintNode::Pattern(pattern_constraint),
+                is_positive: true,
+            });
         pattern_constraint
     }
 
@@ -338,6 +366,7 @@ impl<'db> SemanticIndexBuilder<'db> {
                 // note that the "bound" on the typevar is a totally different thing than whether
                 // or not a name is "bound" by a typevar declaration; the latter is always true.
                 self.mark_symbol_bound(symbol);
+                self.mark_symbol_declared(symbol);
                 if let Some(bounds) = bound {
                     self.visit_expr(bounds);
                 }
@@ -416,7 +445,7 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.pop_scope();
     }
 
-    fn declare_parameter(&mut self, parameter: AnyParameterRef) {
+    fn declare_parameter(&mut self, parameter: AnyParameterRef<'db>) {
         let symbol = self.add_symbol(parameter.name().id().clone());
 
         let definition = self.add_definition(symbol, parameter);
@@ -590,24 +619,48 @@ where
             }
             ast::Stmt::Assign(node) => {
                 debug_assert_eq!(&self.current_assignments, &[]);
+
                 self.visit_expr(&node.value);
-                self.add_standalone_expression(&node.value);
-                for (target_index, target) in node.targets.iter().enumerate() {
-                    let kind = match target {
-                        ast::Expr::List(_) | ast::Expr::Tuple(_) => Some(AssignmentKind::Sequence),
-                        ast::Expr::Name(_) => Some(AssignmentKind::Name),
+                let value = self.add_standalone_expression(&node.value);
+
+                for target in &node.targets {
+                    // We only handle assignments to names and unpackings here, other targets like
+                    // attribute and subscript are handled separately as they don't create a new
+                    // definition.
+                    let current_assignment = match target {
+                        ast::Expr::List(_) | ast::Expr::Tuple(_) => {
+                            Some(CurrentAssignment::Assign {
+                                node,
+                                first: true,
+                                unpack: Some(Unpack::new(
+                                    self.db,
+                                    self.file,
+                                    self.current_scope(),
+                                    #[allow(unsafe_code)]
+                                    unsafe {
+                                        AstNodeRef::new(self.module.clone(), target)
+                                    },
+                                    value,
+                                    countme::Count::default(),
+                                )),
+                            })
+                        }
+                        ast::Expr::Name(_) => Some(CurrentAssignment::Assign {
+                            node,
+                            unpack: None,
+                            first: false,
+                        }),
                         _ => None,
                     };
-                    if let Some(kind) = kind {
-                        self.push_assignment(CurrentAssignment::Assign {
-                            assignment: node,
-                            target_index,
-                            kind,
-                        });
+
+                    if let Some(current_assignment) = current_assignment {
+                        self.push_assignment(current_assignment);
                     }
+
                     self.visit_expr(target);
-                    if kind.is_some() {
-                        // only need to pop in the case where we pushed something
+
+                    if current_assignment.is_some() {
+                        // Only need to pop in the case where we pushed something
                         self.pop_assignment();
                     }
                 }
@@ -639,7 +692,8 @@ where
             ast::Stmt::If(node) => {
                 self.visit_expr(&node.test);
                 let pre_if = self.flow_snapshot();
-                self.add_expression_constraint(&node.test);
+                let constraint = self.record_expression_constraint(&node.test);
+                let mut constraints = vec![constraint];
                 self.visit_body(&node.body);
                 let mut post_clauses: Vec<FlowSnapshot> = vec![];
                 for clause in &node.elif_else_clauses {
@@ -649,7 +703,14 @@ where
                     // we can only take an elif/else branch if none of the previous ones were
                     // taken, so the block entry state is always `pre_if`
                     self.flow_restore(pre_if.clone());
-                    self.visit_elif_else_clause(clause);
+                    for constraint in &constraints {
+                        self.record_negated_constraint(*constraint);
+                    }
+                    if let Some(elif_test) = &clause.test {
+                        self.visit_expr(elif_test);
+                        constraints.push(self.record_expression_constraint(elif_test));
+                    }
+                    self.visit_body(&clause.body);
                 }
                 for post_clause_state in post_clauses {
                     self.flow_merge(post_clause_state);
@@ -697,12 +758,20 @@ where
                     self.flow_merge(break_state);
                 }
             }
-            ast::Stmt::With(ast::StmtWith { items, body, .. }) => {
+            ast::Stmt::With(ast::StmtWith {
+                items,
+                body,
+                is_async,
+                ..
+            }) => {
                 for item in items {
                     self.visit_expr(&item.context_expr);
                     if let Some(optional_vars) = item.optional_vars.as_deref() {
                         self.add_standalone_expression(&item.context_expr);
-                        self.push_assignment(item.into());
+                        self.push_assignment(CurrentAssignment::WithItem {
+                            item,
+                            is_async: *is_async,
+                        });
                         self.visit_expr(optional_vars);
                         self.pop_assignment();
                     }
@@ -918,20 +987,26 @@ where
                 };
                 let symbol = self.add_symbol(id.clone());
 
+                if is_use {
+                    self.mark_symbol_used(symbol);
+                    let use_id = self.current_ast_ids().record_use(expr);
+                    self.current_use_def_map_mut().record_use(symbol, use_id);
+                }
+
                 if is_definition {
-                    match self.current_assignment().copied() {
+                    match self.current_assignment() {
                         Some(CurrentAssignment::Assign {
-                            assignment,
-                            target_index,
-                            kind,
+                            node,
+                            first,
+                            unpack,
                         }) => {
                             self.add_definition(
                                 symbol,
                                 AssignmentDefinitionNodeRef {
-                                    assignment,
-                                    target_index,
+                                    unpack,
+                                    value: &node.value,
                                     name: name_node,
-                                    kind,
+                                    first,
                                 },
                             );
                         }
@@ -968,12 +1043,13 @@ where
                                 },
                             );
                         }
-                        Some(CurrentAssignment::WithItem(with_item)) => {
+                        Some(CurrentAssignment::WithItem { item, is_async }) => {
                             self.add_definition(
                                 symbol,
                                 WithItemDefinitionNodeRef {
-                                    node: with_item,
+                                    node: item,
                                     target: name_node,
+                                    is_async,
                                 },
                             );
                         }
@@ -981,10 +1057,9 @@ where
                     }
                 }
 
-                if is_use {
-                    self.mark_symbol_used(symbol);
-                    let use_id = self.current_ast_ids().record_use(expr);
-                    self.current_use_def_map_mut().record_use(symbol, use_id);
+                if let Some(CurrentAssignment::Assign { first, .. }) = self.current_assignment_mut()
+                {
+                    *first = false;
                 }
 
                 walk_expr(self, expr);
@@ -1027,10 +1102,13 @@ where
                 // AST inspection, so we can't simplify here, need to record test expression for
                 // later checking)
                 self.visit_expr(test);
+                let constraint = self.record_expression_constraint(test);
                 let pre_if = self.flow_snapshot();
                 self.visit_expr(body);
                 let post_body = self.flow_snapshot();
                 self.flow_restore(pre_if);
+
+                self.record_negated_constraint(constraint);
                 self.visit_expr(orelse);
                 self.flow_merge(post_body);
             }
@@ -1084,6 +1162,33 @@ where
                     },
                 );
             }
+            ast::Expr::BoolOp(ast::ExprBoolOp {
+                values,
+                range: _,
+                op,
+            }) => {
+                // TODO detect statically known truthy or falsy values (via type inference, not naive
+                // AST inspection, so we can't simplify here, need to record test expression for
+                // later checking)
+                let mut snapshots = vec![];
+
+                for (index, value) in values.iter().enumerate() {
+                    self.visit_expr(value);
+                    // In the last value we don't need to take a snapshot nor add a constraint
+                    if index < values.len() - 1 {
+                        // Snapshot is taken after visiting the expression but before adding the constraint.
+                        snapshots.push(self.flow_snapshot());
+                        let constraint = self.build_constraint(value);
+                        match op {
+                            BoolOp::And => self.record_constraint(constraint),
+                            BoolOp::Or => self.record_negated_constraint(constraint),
+                        }
+                    }
+                }
+                for snapshot in snapshots {
+                    self.flow_merge(snapshot);
+                }
+            }
             _ => {
                 walk_expr(self, expr);
             }
@@ -1156,9 +1261,9 @@ where
 #[derive(Copy, Clone, Debug, PartialEq)]
 enum CurrentAssignment<'a> {
     Assign {
-        assignment: &'a ast::StmtAssign,
-        target_index: usize,
-        kind: AssignmentKind,
+        node: &'a ast::StmtAssign,
+        first: bool,
+        unpack: Option<Unpack<'a>>,
     },
     AnnAssign(&'a ast::StmtAnnAssign),
     AugAssign(&'a ast::StmtAugAssign),
@@ -1168,7 +1273,10 @@ enum CurrentAssignment<'a> {
         node: &'a ast::Comprehension,
         first: bool,
     },
-    WithItem(&'a ast::WithItem),
+    WithItem {
+        item: &'a ast::WithItem,
+        is_async: bool,
+    },
 }
 
 impl<'a> From<&'a ast::StmtAnnAssign> for CurrentAssignment<'a> {
@@ -1195,12 +1303,6 @@ impl<'a> From<&'a ast::ExprNamed> for CurrentAssignment<'a> {
     }
 }
 
-impl<'a> From<&'a ast::WithItem> for CurrentAssignment<'a> {
-    fn from(value: &'a ast::WithItem) -> Self {
-        Self::WithItem(value)
-    }
-}
-
 struct CurrentMatchCase<'a> {
     /// The pattern that's part of the current match case.
     pattern: &'a ast::Pattern,

crates/red_knot_python_semantic/src/semantic_index/constraint.rs

@@ -7,7 +7,13 @@ use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{FileScopeId, ScopeId};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub(crate) enum Constraint<'db> {
+pub(crate) struct Constraint<'db> {
+    pub(crate) node: ConstraintNode<'db>,
+    pub(crate) is_positive: bool,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum ConstraintNode<'db> {
     Expression(Expression<'db>),
     Pattern(PatternConstraint<'db>),
 }

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -6,8 +6,22 @@ use crate::ast_node_ref::AstNodeRef;
 use crate::module_resolver::file_to_module;
 use crate::node_key::NodeKey;
 use crate::semantic_index::symbol::{FileScopeId, ScopeId, ScopedSymbolId};
+use crate::unpack::Unpack;
 use crate::Db;
 
+/// A definition of a symbol.
+///
+/// ## Module-local type
+/// This type should not be used as part of any cross-module API because
+/// it holds a reference to the AST node. Range-offset changes
+/// then propagate through all usages, and deserialization requires
+/// reparsing the entire module.
+///
+/// E.g. don't use this type in:
+///
+/// * a return type of a cross-module query
+/// * a field of a type that is a return type of a cross-module query
+/// * an argument of a cross-module query
 #[salsa::tracked]
 pub struct Definition<'db> {
     /// The file in which the definition occurs.
@@ -24,7 +38,7 @@ pub struct Definition<'db> {
 
     #[no_eq]
     #[return_ref]
-    pub(crate) kind: DefinitionKind,
+    pub(crate) kind: DefinitionKind<'db>,
 
     #[no_eq]
     count: countme::Count<Definition<'static>>,
@@ -166,16 +180,17 @@ pub(crate) struct ImportFromDefinitionNodeRef<'a> {
 
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct AssignmentDefinitionNodeRef<'a> {
-    pub(crate) assignment: &'a ast::StmtAssign,
-    pub(crate) target_index: usize,
+    pub(crate) unpack: Option<Unpack<'a>>,
+    pub(crate) value: &'a ast::Expr,
     pub(crate) name: &'a ast::ExprName,
-    pub(crate) kind: AssignmentKind,
+    pub(crate) first: bool,
 }
 
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct WithItemDefinitionNodeRef<'a> {
     pub(crate) node: &'a ast::WithItem,
     pub(crate) target: &'a ast::ExprName,
+    pub(crate) is_async: bool,
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -210,9 +225,9 @@ pub(crate) struct MatchPatternDefinitionNodeRef<'a> {
     pub(crate) index: u32,
 }
 
-impl DefinitionNodeRef<'_> {
+impl<'db> DefinitionNodeRef<'db> {
     #[allow(unsafe_code)]
-    pub(super) unsafe fn into_owned(self, parsed: ParsedModule) -> DefinitionKind {
+    pub(super) unsafe fn into_owned(self, parsed: ParsedModule) -> DefinitionKind<'db> {
         match self {
             DefinitionNodeRef::Import(alias) => {
                 DefinitionKind::Import(AstNodeRef::new(parsed, alias))
@@ -233,15 +248,15 @@ impl DefinitionNodeRef<'_> {
                 DefinitionKind::NamedExpression(AstNodeRef::new(parsed, named))
             }
             DefinitionNodeRef::Assignment(AssignmentDefinitionNodeRef {
-                assignment,
-                target_index,
+                unpack,
+                value,
                 name,
-                kind,
+                first,
             }) => DefinitionKind::Assignment(AssignmentDefinitionKind {
-                assignment: AstNodeRef::new(parsed.clone(), assignment),
-                target_index,
+                target: TargetKind::from(unpack),
+                value: AstNodeRef::new(parsed.clone(), value),
                 name: AstNodeRef::new(parsed, name),
-                kind,
+                first,
             }),
             DefinitionNodeRef::AnnotatedAssignment(assign) => {
                 DefinitionKind::AnnotatedAssignment(AstNodeRef::new(parsed, assign))
@@ -277,12 +292,15 @@ impl DefinitionNodeRef<'_> {
                     DefinitionKind::ParameterWithDefault(AstNodeRef::new(parsed, parameter))
                 }
             },
-            DefinitionNodeRef::WithItem(WithItemDefinitionNodeRef { node, target }) => {
-                DefinitionKind::WithItem(WithItemDefinitionKind {
-                    node: AstNodeRef::new(parsed.clone(), node),
-                    target: AstNodeRef::new(parsed, target),
-                })
-            }
+            DefinitionNodeRef::WithItem(WithItemDefinitionNodeRef {
+                node,
+                target,
+                is_async,
+            }) => DefinitionKind::WithItem(WithItemDefinitionKind {
+                node: AstNodeRef::new(parsed.clone(), node),
+                target: AstNodeRef::new(parsed, target),
+                is_async,
+            }),
             DefinitionNodeRef::MatchPattern(MatchPatternDefinitionNodeRef {
                 pattern,
                 identifier,
@@ -296,7 +314,7 @@ impl DefinitionNodeRef<'_> {
                 handler,
                 is_star,
             }) => DefinitionKind::ExceptHandler(ExceptHandlerDefinitionKind {
-                handler: AstNodeRef::new(parsed.clone(), handler),
+                handler: AstNodeRef::new(parsed, handler),
                 is_star,
             }),
         }
@@ -312,10 +330,10 @@ impl DefinitionNodeRef<'_> {
             Self::Class(node) => node.into(),
             Self::NamedExpression(node) => node.into(),
             Self::Assignment(AssignmentDefinitionNodeRef {
-                assignment: _,
-                target_index: _,
+                value: _,
+                unpack: _,
                 name,
-                kind: _,
+                first: _,
             }) => name.into(),
             Self::AnnotatedAssignment(node) => node.into(),
             Self::AugmentedAssignment(node) => node.into(),
@@ -329,7 +347,11 @@ impl DefinitionNodeRef<'_> {
                 ast::AnyParameterRef::Variadic(parameter) => parameter.into(),
                 ast::AnyParameterRef::NonVariadic(parameter) => parameter.into(),
             },
-            Self::WithItem(WithItemDefinitionNodeRef { node: _, target }) => target.into(),
+            Self::WithItem(WithItemDefinitionNodeRef {
+                node: _,
+                target,
+                is_async: _,
+            }) => target.into(),
             Self::MatchPattern(MatchPatternDefinitionNodeRef { identifier, .. }) => {
                 identifier.into()
             }
@@ -374,13 +396,13 @@ impl DefinitionCategory {
 }
 
 #[derive(Clone, Debug)]
-pub enum DefinitionKind {
+pub enum DefinitionKind<'db> {
     Import(AstNodeRef<ast::Alias>),
     ImportFrom(ImportFromDefinitionKind),
     Function(AstNodeRef<ast::StmtFunctionDef>),
     Class(AstNodeRef<ast::StmtClassDef>),
     NamedExpression(AstNodeRef<ast::ExprNamed>),
-    Assignment(AssignmentDefinitionKind),
+    Assignment(AssignmentDefinitionKind<'db>),
     AnnotatedAssignment(AstNodeRef<ast::StmtAnnAssign>),
     AugmentedAssignment(AstNodeRef<ast::StmtAugAssign>),
     For(ForStmtDefinitionKind),
@@ -392,7 +414,7 @@ pub enum DefinitionKind {
     ExceptHandler(ExceptHandlerDefinitionKind),
 }
 
-impl DefinitionKind {
+impl DefinitionKind<'_> {
     pub(crate) fn category(&self) -> DefinitionCategory {
         match self {
             // functions, classes, and imports always bind, and we consider them declarations
@@ -437,6 +459,21 @@ impl DefinitionKind {
     }
 }
 
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub(crate) enum TargetKind<'db> {
+    Sequence(Unpack<'db>),
+    Name,
+}
+
+impl<'db> From<Option<Unpack<'db>>> for TargetKind<'db> {
+    fn from(value: Option<Unpack<'db>>) -> Self {
+        match value {
+            Some(unpack) => TargetKind::Sequence(unpack),
+            None => TargetKind::Name,
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 #[allow(dead_code)]
 pub struct MatchPatternDefinitionKind {
@@ -498,42 +535,36 @@ impl ImportFromDefinitionKind {
 }
 
 #[derive(Clone, Debug)]
-pub struct AssignmentDefinitionKind {
-    assignment: AstNodeRef<ast::StmtAssign>,
-    target_index: usize,
+pub struct AssignmentDefinitionKind<'db> {
+    target: TargetKind<'db>,
+    value: AstNodeRef<ast::Expr>,
     name: AstNodeRef<ast::ExprName>,
-    kind: AssignmentKind,
+    first: bool,
 }
 
-impl AssignmentDefinitionKind {
-    pub(crate) fn value(&self) -> &ast::Expr {
-        &self.assignment.node().value
+impl<'db> AssignmentDefinitionKind<'db> {
+    pub(crate) fn target(&self) -> TargetKind<'db> {
+        self.target
     }
 
-    pub(crate) fn target(&self) -> &ast::Expr {
-        &self.assignment.node().targets[self.target_index]
+    pub(crate) fn value(&self) -> &ast::Expr {
+        self.value.node()
     }
 
     pub(crate) fn name(&self) -> &ast::ExprName {
         self.name.node()
     }
 
-    pub(crate) fn kind(&self) -> AssignmentKind {
-        self.kind
+    pub(crate) fn is_first(&self) -> bool {
+        self.first
     }
 }
 
-/// The kind of assignment target expression.
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub enum AssignmentKind {
-    Sequence,
-    Name,
-}
-
 #[derive(Clone, Debug)]
 pub struct WithItemDefinitionKind {
     node: AstNodeRef<ast::WithItem>,
     target: AstNodeRef<ast::ExprName>,
+    is_async: bool,
 }
 
 impl WithItemDefinitionKind {
@@ -544,6 +575,10 @@ impl WithItemDefinitionKind {
     pub(crate) fn target(&self) -> &ast::ExprName {
         self.target.node()
     }
+
+    pub(crate) const fn is_async(&self) -> bool {
+        self.is_async
+    }
 }
 
 #[derive(Clone, Debug)]

crates/red_knot_python_semantic/src/semantic_index/expression.rs

@@ -8,6 +8,18 @@ use salsa;
 /// An independently type-inferable expression.
 ///
 /// Includes constraint expressions (e.g. if tests) and the RHS of an unpacking assignment.
+///
+/// ## Module-local type
+/// This type should not be used as part of any cross-module API because
+/// it holds a reference to the AST node. Range-offset changes
+/// then propagate through all usages, and deserialization requires
+/// reparsing the entire module.
+///
+/// E.g. don't use this type in:
+///
+/// * a return type of a cross-module query
+/// * a field of a type that is a return type of a cross-module query
+/// * an argument of a cross-module query
 #[salsa::tracked]
 pub(crate) struct Expression<'db> {
     /// The file in which the expression occurs.

crates/red_knot_python_semantic/src/semantic_index/symbol.rs

@@ -47,17 +47,27 @@ impl Symbol {
     pub fn is_bound(&self) -> bool {
         self.flags.contains(SymbolFlags::IS_BOUND)
     }
+
+    /// Is the symbol declared in its containing scope?
+    pub fn is_declared(&self) -> bool {
+        self.flags.contains(SymbolFlags::IS_DECLARED)
+    }
 }
 
 bitflags! {
+    /// Flags that can be queried to obtain information about a symbol in a given scope.
+    ///
+    /// See the doc-comment at the top of [`super::use_def`] for explanations of what it
+    /// means for a symbol to be *bound* as opposed to *declared*.
     #[derive(Copy, Clone, Debug, Eq, PartialEq)]
     struct SymbolFlags: u8 {
         const IS_USED         = 1 << 0;
-        const IS_BOUND      = 1 << 1;
+        const IS_BOUND        = 1 << 1;
+        const IS_DECLARED     = 1 << 2;
         /// TODO: This flag is not yet set by anything
-        const MARKED_GLOBAL   = 1 << 2;
+        const MARKED_GLOBAL   = 1 << 3;
         /// TODO: This flag is not yet set by anything
-        const MARKED_NONLOCAL = 1 << 3;
+        const MARKED_NONLOCAL = 1 << 4;
     }
 }
 
@@ -93,14 +103,10 @@ pub struct ScopedSymbolId;
 pub struct ScopeId<'db> {
     #[id]
     pub file: File,
+
     #[id]
     pub file_scope_id: FileScopeId,
 
-    /// The node that introduces this scope.
-    #[no_eq]
-    #[return_ref]
-    pub node: NodeWithScopeKind,
-
     #[no_eq]
     count: countme::Count<ScopeId<'static>>,
 }
@@ -121,6 +127,14 @@ impl<'db> ScopeId<'db> {
         )
     }
 
+    pub(crate) fn node(self, db: &dyn Db) -> &NodeWithScopeKind {
+        self.scope(db).node()
+    }
+
+    pub(crate) fn scope(self, db: &dyn Db) -> &Scope {
+        semantic_index(db, self.file(db)).scope(self.file_scope_id(db))
+    }
+
     #[cfg(test)]
     pub(crate) fn name(self, db: &'db dyn Db) -> &'db str {
         match self.node(db) {
@@ -159,10 +173,10 @@ impl FileScopeId {
     }
 }
 
-#[derive(Debug, Eq, PartialEq)]
+#[derive(Debug)]
 pub struct Scope {
     pub(super) parent: Option<FileScopeId>,
-    pub(super) kind: ScopeKind,
+    pub(super) node: NodeWithScopeKind,
     pub(super) descendents: Range<FileScopeId>,
 }
 
@@ -171,8 +185,12 @@ impl Scope {
         self.parent
     }
 
+    pub fn node(&self) -> &NodeWithScopeKind {
+        &self.node
+    }
+
     pub fn kind(&self) -> ScopeKind {
-        self.kind
+        self.node().scope_kind()
     }
 }
 
@@ -298,6 +316,10 @@ impl SymbolTableBuilder {
         self.table.symbols[id].insert_flags(SymbolFlags::IS_BOUND);
     }
 
+    pub(super) fn mark_symbol_declared(&mut self, id: ScopedSymbolId) {
+        self.table.symbols[id].insert_flags(SymbolFlags::IS_DECLARED);
+    }
+
     pub(super) fn mark_symbol_used(&mut self, id: ScopedSymbolId) {
         self.table.symbols[id].insert_flags(SymbolFlags::IS_USED);
     }
@@ -362,21 +384,6 @@ impl NodeWithScopeRef<'_> {
         }
     }
 
-    pub(super) fn scope_kind(self) -> ScopeKind {
-        match self {
-            NodeWithScopeRef::Module => ScopeKind::Module,
-            NodeWithScopeRef::Class(_) => ScopeKind::Class,
-            NodeWithScopeRef::Function(_) => ScopeKind::Function,
-            NodeWithScopeRef::Lambda(_) => ScopeKind::Function,
-            NodeWithScopeRef::FunctionTypeParameters(_)
-            | NodeWithScopeRef::ClassTypeParameters(_) => ScopeKind::Annotation,
-            NodeWithScopeRef::ListComprehension(_)
-            | NodeWithScopeRef::SetComprehension(_)
-            | NodeWithScopeRef::DictComprehension(_)
-            | NodeWithScopeRef::GeneratorExpression(_) => ScopeKind::Comprehension,
-        }
-    }
-
     pub(crate) fn node_key(self) -> NodeWithScopeKey {
         match self {
             NodeWithScopeRef::Module => NodeWithScopeKey::Module,
@@ -424,6 +431,36 @@ pub enum NodeWithScopeKind {
     GeneratorExpression(AstNodeRef<ast::ExprGenerator>),
 }
 
+impl NodeWithScopeKind {
+    pub(super) const fn scope_kind(&self) -> ScopeKind {
+        match self {
+            Self::Module => ScopeKind::Module,
+            Self::Class(_) => ScopeKind::Class,
+            Self::Function(_) => ScopeKind::Function,
+            Self::Lambda(_) => ScopeKind::Function,
+            Self::FunctionTypeParameters(_) | Self::ClassTypeParameters(_) => ScopeKind::Annotation,
+            Self::ListComprehension(_)
+            | Self::SetComprehension(_)
+            | Self::DictComprehension(_)
+            | Self::GeneratorExpression(_) => ScopeKind::Comprehension,
+        }
+    }
+
+    pub fn expect_class(&self) -> &ast::StmtClassDef {
+        match self {
+            Self::Class(class) => class.node(),
+            _ => panic!("expected class"),
+        }
+    }
+
+    pub fn expect_function(&self) -> &ast::StmtFunctionDef {
+        match self {
+            Self::Function(function) => function.node(),
+            _ => panic!("expected function"),
+        }
+    }
+}
+
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
 pub(crate) enum NodeWithScopeKey {
     Module,

crates/red_knot_python_semantic/src/semantic_index/use_def.rs

@@ -228,6 +228,7 @@ use self::symbol_state::{
 use crate::semantic_index::ast_ids::ScopedUseId;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::symbol::ScopedSymbolId;
+use crate::symbol::Boundness;
 use ruff_index::IndexVec;
 use rustc_hash::FxHashMap;
 
@@ -274,8 +275,12 @@ impl<'db> UseDefMap<'db> {
         self.bindings_iterator(&self.bindings_by_use[use_id])
     }
 
-    pub(crate) fn use_may_be_unbound(&self, use_id: ScopedUseId) -> bool {
-        self.bindings_by_use[use_id].may_be_unbound()
+    pub(crate) fn use_boundness(&self, use_id: ScopedUseId) -> Boundness {
+        if self.bindings_by_use[use_id].may_be_unbound() {
+            Boundness::MayBeUnbound
+        } else {
+            Boundness::Bound
+        }
     }
 
     pub(crate) fn public_bindings(
@@ -285,8 +290,12 @@ impl<'db> UseDefMap<'db> {
         self.bindings_iterator(self.public_symbols[symbol].bindings())
     }
 
-    pub(crate) fn public_may_be_unbound(&self, symbol: ScopedSymbolId) -> bool {
-        self.public_symbols[symbol].may_be_unbound()
+    pub(crate) fn public_boundness(&self, symbol: ScopedSymbolId) -> Boundness {
+        if self.public_symbols[symbol].may_be_unbound() {
+            Boundness::MayBeUnbound
+        } else {
+            Boundness::Bound
+        }
     }
 
     pub(crate) fn bindings_at_declaration(

crates/red_knot_python_semantic/src/semantic_model.rs

@@ -8,7 +8,7 @@ use crate::module_name::ModuleName;
 use crate::module_resolver::{resolve_module, Module};
 use crate::semantic_index::ast_ids::HasScopedAstId;
 use crate::semantic_index::semantic_index;
-use crate::types::{binding_ty, global_symbol_ty, infer_scope_types, Type};
+use crate::types::{binding_ty, infer_scope_types, Type};
 use crate::Db;
 
 pub struct SemanticModel<'db> {
@@ -38,10 +38,6 @@ impl<'db> SemanticModel<'db> {
     pub fn resolve_module(&self, module_name: &ModuleName) -> Option<Module> {
         resolve_module(self.db, module_name)
     }
-
-    pub fn global_symbol_ty(&self, module: &Module, symbol_name: &str) -> Type<'db> {
-        global_symbol_ty(self.db, module.file(), symbol_name)
-    }
 }
 
 pub trait HasTy {

crates/red_knot_python_semantic/src/stdlib.rs

@@ -2,7 +2,8 @@ use crate::module_name::ModuleName;
 use crate::module_resolver::resolve_module;
 use crate::semantic_index::global_scope;
 use crate::semantic_index::symbol::ScopeId;
-use crate::types::{global_symbol_ty, Type};
+use crate::symbol::Symbol;
+use crate::types::global_symbol;
 use crate::Db;
 
 /// Enumeration of various core stdlib modules, for which we have dedicated Salsa queries.
@@ -12,6 +13,7 @@ enum CoreStdlibModule {
     Types,
     Typeshed,
     TypingExtensions,
+    Typing,
 }
 
 impl CoreStdlibModule {
@@ -19,6 +21,7 @@ impl CoreStdlibModule {
         let module_name = match self {
             Self::Builtins => "builtins",
             Self::Types => "types",
+            Self::Typing => "typing",
             Self::Typeshed => "_typeshed",
             Self::TypingExtensions => "typing_extensions",
         };
@@ -29,54 +32,55 @@ impl CoreStdlibModule {
 
 /// Lookup the type of `symbol` in a given core module
 ///
-/// Returns `Unbound` if the given core module cannot be resolved for some reason
-fn core_module_symbol_ty<'db>(
+/// Returns `Symbol::Unbound` if the given core module cannot be resolved for some reason
+fn core_module_symbol<'db>(
     db: &'db dyn Db,
     core_module: CoreStdlibModule,
     symbol: &str,
-) -> Type<'db> {
+) -> Symbol<'db> {
     resolve_module(db, &core_module.name())
-        .map(|module| global_symbol_ty(db, module.file(), symbol))
-        .map(|ty| {
-            if ty.is_unbound() {
-                ty
-            } else {
-                ty.replace_unbound_with(db, Type::Never)
-            }
-        })
-        .unwrap_or(Type::Unbound)
+        .map(|module| global_symbol(db, module.file(), symbol))
+        .unwrap_or(Symbol::Unbound)
 }
 
 /// Lookup the type of `symbol` in the builtins namespace.
 ///
-/// Returns `Unbound` if the `builtins` module isn't available for some reason.
+/// Returns `Symbol::Unbound` if the `builtins` module isn't available for some reason.
 #[inline]
-pub(crate) fn builtins_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db> {
-    core_module_symbol_ty(db, CoreStdlibModule::Builtins, symbol)
+pub(crate) fn builtins_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
+    core_module_symbol(db, CoreStdlibModule::Builtins, symbol)
 }
 
 /// Lookup the type of `symbol` in the `types` module namespace.
 ///
-/// Returns `Unbound` if the `types` module isn't available for some reason.
+/// Returns `Symbol::Unbound` if the `types` module isn't available for some reason.
 #[inline]
-pub(crate) fn types_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db> {
-    core_module_symbol_ty(db, CoreStdlibModule::Types, symbol)
+pub(crate) fn types_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
+    core_module_symbol(db, CoreStdlibModule::Types, symbol)
 }
 
+/// Lookup the type of `symbol` in the `typing` module namespace.
+///
+/// Returns `Symbol::Unbound` if the `typing` module isn't available for some reason.
+#[inline]
+#[allow(dead_code)] // currently only used in tests
+pub(crate) fn typing_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
+    core_module_symbol(db, CoreStdlibModule::Typing, symbol)
+}
 /// Lookup the type of `symbol` in the `_typeshed` module namespace.
 ///
-/// Returns `Unbound` if the `_typeshed` module isn't available for some reason.
+/// Returns `Symbol::Unbound` if the `_typeshed` module isn't available for some reason.
 #[inline]
-pub(crate) fn typeshed_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db> {
-    core_module_symbol_ty(db, CoreStdlibModule::Typeshed, symbol)
+pub(crate) fn typeshed_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
+    core_module_symbol(db, CoreStdlibModule::Typeshed, symbol)
 }
 
 /// Lookup the type of `symbol` in the `typing_extensions` module namespace.
 ///
-/// Returns `Unbound` if the `typing_extensions` module isn't available for some reason.
+/// Returns `Symbol::Unbound` if the `typing_extensions` module isn't available for some reason.
 #[inline]
-pub(crate) fn typing_extensions_symbol_ty<'db>(db: &'db dyn Db, symbol: &str) -> Type<'db> {
-    core_module_symbol_ty(db, CoreStdlibModule::TypingExtensions, symbol)
+pub(crate) fn typing_extensions_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
+    core_module_symbol(db, CoreStdlibModule::TypingExtensions, symbol)
 }
 
 /// Get the scope of a core stdlib module.

crates/red_knot_python_semantic/src/symbol.rs

@@ -0,0 +1,88 @@
+use crate::{
+    types::{Type, UnionType},
+    Db,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum Boundness {
+    Bound,
+    MayBeUnbound,
+}
+
+/// The result of a symbol lookup, which can either be a (possibly unbound) type
+/// or a completely unbound symbol.
+///
+/// Consider this example:
+/// ```py
+/// bound = 1
+///
+/// if flag:
+///     maybe_unbound = 2
+/// ```
+///
+/// If we look up symbols in this scope, we would get the following results:
+/// ```rs
+/// bound:          Symbol::Type(Type::IntLiteral(1), Boundness::Bound),
+/// maybe_unbound:  Symbol::Type(Type::IntLiteral(2), Boundness::MayBeUnbound),
+/// non_existent:   Symbol::Unbound,
+/// ```
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) enum Symbol<'db> {
+    Type(Type<'db>, Boundness),
+    Unbound,
+}
+
+impl<'db> Symbol<'db> {
+    pub(crate) fn is_unbound(&self) -> bool {
+        matches!(self, Symbol::Unbound)
+    }
+
+    pub(crate) fn may_be_unbound(&self) -> bool {
+        match self {
+            Symbol::Type(_, Boundness::MayBeUnbound) | Symbol::Unbound => true,
+            Symbol::Type(_, Boundness::Bound) => false,
+        }
+    }
+
+    pub(crate) fn unwrap_or_unknown(&self) -> Type<'db> {
+        match self {
+            Symbol::Type(ty, _) => *ty,
+            Symbol::Unbound => Type::Unknown,
+        }
+    }
+
+    pub(crate) fn as_type(&self) -> Option<Type<'db>> {
+        match self {
+            Symbol::Type(ty, _) => Some(*ty),
+            Symbol::Unbound => None,
+        }
+    }
+
+    #[cfg(test)]
+    #[track_caller]
+    pub(crate) fn expect_type(self) -> Type<'db> {
+        self.as_type()
+            .expect("Expected a (possibly unbound) type, not an unbound symbol")
+    }
+
+    #[must_use]
+    pub(crate) fn replace_unbound_with(
+        self,
+        db: &'db dyn Db,
+        replacement: &Symbol<'db>,
+    ) -> Symbol<'db> {
+        match replacement {
+            Symbol::Type(replacement, _) => Symbol::Type(
+                match self {
+                    Symbol::Type(ty, Boundness::Bound) => ty,
+                    Symbol::Type(ty, Boundness::MayBeUnbound) => {
+                        UnionType::from_elements(db, [*replacement, ty])
+                    }
+                    Symbol::Unbound => *replacement,
+                },
+                Boundness::Bound,
+            ),
+            Symbol::Unbound => self,
+        }
+    }
+}

crates/red_knot_python_semantic/src/types/builder.rs

@@ -25,12 +25,11 @@
 //!   * No type in an intersection can be a supertype of any other type in the intersection (just
 //!     eliminate the supertype from the intersection).
 //!   * An intersection containing two non-overlapping types should simplify to [`Type::Never`].
-use crate::types::{IntersectionType, Type, UnionType};
+
+use crate::types::{InstanceType, IntersectionType, KnownClass, Type, UnionType};
 use crate::{Db, FxOrderSet};
 use smallvec::SmallVec;
 
-use super::KnownClass;
-
 pub(crate) struct UnionBuilder<'db> {
     elements: Vec<Type<'db>>,
     db: &'db dyn Db,
@@ -80,7 +79,6 @@ impl<'db> UnionBuilder<'db> {
                         to_remove.push(index);
                     }
                 }
-
                 match to_remove[..] {
                     [] => self.elements.push(to_add),
                     [index] => self.elements[index] = to_add,
@@ -103,7 +101,6 @@ impl<'db> UnionBuilder<'db> {
                 }
             }
         }
-
         self
     }
 
@@ -173,14 +170,37 @@ impl<'db> IntersectionBuilder<'db> {
     pub(crate) fn add_negative(mut self, ty: Type<'db>) -> Self {
         // See comments above in `add_positive`; this is just the negated version.
         if let Type::Union(union) = ty {
-            union
-                .elements(self.db)
+            for elem in union.elements(self.db) {
+                self = self.add_negative(*elem);
+            }
+            self
+        } else if let Type::Intersection(intersection) = ty {
+            // (A | B) & ~(C & ~D)
+            // -> (A | B) & (~C | D)
+            // -> ((A | B) & ~C) | ((A | B) & D)
+            // i.e. if we have an intersection of positive constraints C
+            // and negative constraints D, then our new intersection
+            // is (existing & ~C) | (existing & D)
+
+            let positive_side = intersection
+                .positive(self.db)
                 .iter()
-                .map(|elem| self.clone().add_negative(*elem))
-                .fold(IntersectionBuilder::empty(self.db), |mut builder, sub| {
+                // we negate all the positive constraints while distributing
+                .map(|elem| self.clone().add_negative(*elem));
+
+            let negative_side = intersection
+                .negative(self.db)
+                .iter()
+                // all negative constraints end up becoming positive constraints
+                .map(|elem| self.clone().add_positive(*elem));
+
+            positive_side.chain(negative_side).fold(
+                IntersectionBuilder::empty(self.db),
+                |mut builder, sub| {
                     builder.intersections.extend(sub.intersections);
                     builder
-                })
+                },
+            )
         } else {
             for inner in &mut self.intersections {
                 inner.add_negative(self.db, ty);
@@ -226,8 +246,8 @@ impl<'db> InnerIntersectionBuilder<'db> {
             }
         } else {
             // ~Literal[True] & bool = Literal[False]
-            if let Type::Instance(class_type) = new_positive {
-                if class_type.is_known(db, KnownClass::Bool) {
+            if let Type::Instance(InstanceType { class }) = new_positive {
+                if class.is_known(db, KnownClass::Bool) {
                     if let Some(&Type::BooleanLiteral(value)) = self
                         .negative
                         .iter()
@@ -293,12 +313,11 @@ impl<'db> InnerIntersectionBuilder<'db> {
                     self.add_positive(db, *neg);
                 }
             }
-            Type::Unbound => {}
             ty @ (Type::Any | Type::Unknown | Type::Todo) => {
                 // Adding any of these types to the negative side of an intersection
                 // is equivalent to adding it to the positive side. We do this to
                 // simplify the representation.
-                self.positive.insert(ty);
+                self.add_positive(db, ty);
             }
             // ~Literal[True] & bool = Literal[False]
             Type::BooleanLiteral(bool)
@@ -344,15 +363,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
         }
     }
 
-    fn simplify_unbound(&mut self) {
-        if self.positive.contains(&Type::Unbound) {
-            self.positive.retain(Type::is_unbound);
-            self.negative.clear();
-        }
-    }
-
     fn build(mut self, db: &'db dyn Db) -> Type<'db> {
-        self.simplify_unbound();
         match (self.positive.len(), self.negative.len()) {
             (0, 0) => KnownClass::Object.to_instance(db),
             (1, 0) => self.positive[0],
@@ -371,8 +382,10 @@ mod tests {
     use crate::db::tests::TestDb;
     use crate::program::{Program, SearchPathSettings};
     use crate::python_version::PythonVersion;
-    use crate::types::{KnownClass, StringLiteralType, UnionBuilder};
+    use crate::stdlib::typing_symbol;
+    use crate::types::{global_symbol, KnownClass, StringLiteralType, UnionBuilder};
     use crate::ProgramSettings;
+    use ruff_db::files::system_path_to_file;
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
     use test_case::test_case;
 
@@ -561,18 +574,38 @@ mod tests {
         let ta = Type::Any;
         let t1 = Type::IntLiteral(1);
         let t2 = KnownClass::Int.to_instance(&db);
+        // i0 = Any & ~Literal[1]
         let i0 = IntersectionBuilder::new(&db)
             .add_positive(ta)
             .add_negative(t1)
             .build();
-        let intersection = IntersectionBuilder::new(&db)
+        // ta_not_i0 = int & ~(Any & ~Literal[1])
+        // -> int & (~Any  | Literal[1])
+        // (~Any is equivalent to Any)
+        // -> (int & Any) | (int & Literal[1])
+        // -> (int & Any) | Literal[1]
+        let ta_not_i0 = IntersectionBuilder::new(&db)
             .add_positive(t2)
             .add_negative(i0)
-            .build()
-            .expect_intersection();
+            .build();
 
-        assert_eq!(intersection.pos_vec(&db), &[ta, t1]);
-        assert_eq!(intersection.neg_vec(&db), &[]);
+        assert_eq!(ta_not_i0.display(&db).to_string(), "int & Any | Literal[1]");
+    }
+
+    #[test]
+    fn build_intersection_simplify_negative_any() {
+        let db = setup_db();
+
+        let ty = IntersectionBuilder::new(&db)
+            .add_negative(Type::Any)
+            .build();
+        assert_eq!(ty, Type::Any);
+
+        let ty = IntersectionBuilder::new(&db)
+            .add_positive(Type::Never)
+            .add_negative(Type::Any)
+            .build();
+        assert_eq!(ty, Type::Never);
     }
 
     #[test]
@@ -595,12 +628,69 @@ mod tests {
         assert_eq!(i1.pos_vec(&db), &[ta, t1]);
     }
 
+    #[test]
+    fn intersection_negation_distributes_over_union() {
+        let db = setup_db();
+        let st = typing_symbol(&db, "Sized").expect_type().to_instance(&db);
+        let ht = typing_symbol(&db, "Hashable")
+            .expect_type()
+            .to_instance(&db);
+        // sh_t: Sized & Hashable
+        let sh_t = IntersectionBuilder::new(&db)
+            .add_positive(st)
+            .add_positive(ht)
+            .build()
+            .expect_intersection();
+        assert_eq!(sh_t.pos_vec(&db), &[st, ht]);
+        assert_eq!(sh_t.neg_vec(&db), &[]);
+
+        // ~sh_t => ~Sized | ~Hashable
+        let not_s_h_t = IntersectionBuilder::new(&db)
+            .add_negative(Type::Intersection(sh_t))
+            .build()
+            .expect_union();
+
+        // should have as elements: (~Sized),(~Hashable)
+        let not_st = st.negate(&db);
+        let not_ht = ht.negate(&db);
+        assert_eq!(not_s_h_t.elements(&db), &[not_st, not_ht]);
+    }
+
+    #[test]
+    fn mixed_intersection_negation_distributes_over_union() {
+        let db = setup_db();
+        let it = KnownClass::Int.to_instance(&db);
+        let st = typing_symbol(&db, "Sized").expect_type().to_instance(&db);
+        let ht = typing_symbol(&db, "Hashable")
+            .expect_type()
+            .to_instance(&db);
+        // s_not_h_t: Sized & ~Hashable
+        let s_not_h_t = IntersectionBuilder::new(&db)
+            .add_positive(st)
+            .add_negative(ht)
+            .build()
+            .expect_intersection();
+        assert_eq!(s_not_h_t.pos_vec(&db), &[st]);
+        assert_eq!(s_not_h_t.neg_vec(&db), &[ht]);
+
+        // let's build int & ~(Sized & ~Hashable)
+        let tt = IntersectionBuilder::new(&db)
+            .add_positive(it)
+            .add_negative(Type::Intersection(s_not_h_t))
+            .build();
+
+        // int & ~(Sized & ~Hashable)
+        // -> int & (~Sized | Hashable)
+        // -> (int & ~Sized) | (int & Hashable)
+        assert_eq!(tt.display(&db).to_string(), "int & ~Sized | int & Hashable");
+    }
+
     #[test]
     fn build_intersection_self_negation() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
-            .add_negative(Type::None)
+            .add_positive(Type::none(&db))
+            .add_negative(Type::none(&db))
             .build();
 
         assert_eq!(ty, Type::Never);
@@ -610,63 +700,62 @@ mod tests {
     fn build_intersection_simplify_negative_never() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
+            .add_positive(Type::none(&db))
             .add_negative(Type::Never)
             .build();
 
-        assert_eq!(ty, Type::None);
+        assert_eq!(ty, Type::none(&db));
     }
 
     #[test]
     fn build_intersection_simplify_positive_never() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
+            .add_positive(Type::none(&db))
             .add_positive(Type::Never)
             .build();
 
         assert_eq!(ty, Type::Never);
     }
 
-    #[test]
-    fn build_intersection_simplify_positive_unbound() {
-        let db = setup_db();
-        let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::Unbound)
-            .add_positive(Type::IntLiteral(1))
-            .build();
-
-        assert_eq!(ty, Type::Unbound);
-    }
-
-    #[test]
-    fn build_intersection_simplify_negative_unbound() {
-        let db = setup_db();
-        let ty = IntersectionBuilder::new(&db)
-            .add_negative(Type::Unbound)
-            .add_positive(Type::IntLiteral(1))
-            .build();
-
-        assert_eq!(ty, Type::IntLiteral(1));
-    }
-
     #[test]
     fn build_intersection_simplify_negative_none() {
         let db = setup_db();
 
         let ty = IntersectionBuilder::new(&db)
-            .add_negative(Type::None)
+            .add_negative(Type::none(&db))
             .add_positive(Type::IntLiteral(1))
             .build();
         assert_eq!(ty, Type::IntLiteral(1));
 
         let ty = IntersectionBuilder::new(&db)
             .add_positive(Type::IntLiteral(1))
-            .add_negative(Type::None)
+            .add_negative(Type::none(&db))
             .build();
         assert_eq!(ty, Type::IntLiteral(1));
     }
 
+    #[test]
+    fn build_negative_union_de_morgan() {
+        let db = setup_db();
+
+        let union = UnionBuilder::new(&db)
+            .add(Type::IntLiteral(1))
+            .add(Type::IntLiteral(2))
+            .build();
+        assert_eq!(union.display(&db).to_string(), "Literal[1, 2]");
+
+        let ty = IntersectionBuilder::new(&db).add_negative(union).build();
+
+        let expected = IntersectionBuilder::new(&db)
+            .add_negative(Type::IntLiteral(1))
+            .add_negative(Type::IntLiteral(2))
+            .build();
+
+        assert_eq!(ty.display(&db).to_string(), "~Literal[1] & ~Literal[2]");
+        assert_eq!(ty, expected);
+    }
+
     #[test]
     fn build_intersection_simplify_positive_type_and_positive_subtype() {
         let db = setup_db();
@@ -800,7 +889,7 @@ mod tests {
         let db = setup_db();
 
         let t1 = Type::IntLiteral(1);
-        let t2 = Type::None;
+        let t2 = Type::none(&db);
 
         let ty = IntersectionBuilder::new(&db)
             .add_positive(t1)
@@ -918,4 +1007,66 @@ mod tests {
             .build();
         assert_eq!(result, ty);
     }
+
+    #[test]
+    fn build_intersection_of_two_unions_simplify() {
+        let mut db = setup_db();
+        db.write_dedented(
+            "/src/module.py",
+            "
+            class A: ...
+            class B: ...
+            a = A()
+            b = B()
+        ",
+        )
+        .unwrap();
+
+        let file = system_path_to_file(&db, "src/module.py").expect("file to exist");
+
+        let a = global_symbol(&db, file, "a").expect_type();
+        let b = global_symbol(&db, file, "b").expect_type();
+        let union = UnionBuilder::new(&db).add(a).add(b).build();
+        assert_eq!(union.display(&db).to_string(), "A | B");
+        let reversed_union = UnionBuilder::new(&db).add(b).add(a).build();
+        assert_eq!(reversed_union.display(&db).to_string(), "B | A");
+        let intersection = IntersectionBuilder::new(&db)
+            .add_positive(union)
+            .add_positive(reversed_union)
+            .build();
+        assert_eq!(intersection.display(&db).to_string(), "B | A");
+    }
+
+    #[test]
+    fn build_union_of_two_intersections_simplify() {
+        let mut db = setup_db();
+        db.write_dedented(
+            "/src/module.py",
+            "
+            class A: ...
+            class B: ...
+            a = A()
+            b = B()
+        ",
+        )
+        .unwrap();
+
+        let file = system_path_to_file(&db, "src/module.py").expect("file to exist");
+
+        let a = global_symbol(&db, file, "a").expect_type();
+        let b = global_symbol(&db, file, "b").expect_type();
+        let intersection = IntersectionBuilder::new(&db)
+            .add_positive(a)
+            .add_positive(b)
+            .build();
+        let reversed_intersection = IntersectionBuilder::new(&db)
+            .add_positive(b)
+            .add_positive(a)
+            .build();
+        let union = UnionBuilder::new(&db)
+            .add(intersection)
+            .add(reversed_intersection)
+            .build();
+        assert_eq!(union.display(&db).to_string(), "A & B");
+    }
 }

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -1,10 +1,15 @@
+use crate::types::{ClassLiteralType, Type};
+use crate::Db;
+use ruff_db::diagnostic::{Diagnostic, Severity};
 use ruff_db::files::File;
+use ruff_python_ast::{self as ast, AnyNodeRef};
 use ruff_text_size::{Ranged, TextRange};
+use std::borrow::Cow;
 use std::fmt::Formatter;
 use std::ops::Deref;
 use std::sync::Arc;
 
-#[derive(Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq, Clone)]
 pub struct TypeCheckDiagnostic {
     // TODO: Don't use string keys for rules
     pub(super) rule: String,
@@ -27,6 +32,28 @@ impl TypeCheckDiagnostic {
     }
 }
 
+impl Diagnostic for TypeCheckDiagnostic {
+    fn rule(&self) -> &str {
+        TypeCheckDiagnostic::rule(self)
+    }
+
+    fn message(&self) -> Cow<str> {
+        TypeCheckDiagnostic::message(self).into()
+    }
+
+    fn file(&self) -> File {
+        TypeCheckDiagnostic::file(self)
+    }
+
+    fn range(&self) -> Option<TextRange> {
+        Some(Ranged::range(self))
+    }
+
+    fn severity(&self) -> Severity {
+        Severity::Error
+    }
+}
+
 impl Ranged for TypeCheckDiagnostic {
     fn range(&self) -> TextRange {
         self.range
@@ -109,3 +136,191 @@ impl<'a> IntoIterator for &'a TypeCheckDiagnostics {
         self.inner.iter()
     }
 }
+
+pub(super) struct TypeCheckDiagnosticsBuilder<'db> {
+    db: &'db dyn Db,
+    file: File,
+    diagnostics: TypeCheckDiagnostics,
+}
+
+impl<'db> TypeCheckDiagnosticsBuilder<'db> {
+    pub(super) fn new(db: &'db dyn Db, file: File) -> Self {
+        Self {
+            db,
+            file,
+            diagnostics: TypeCheckDiagnostics::new(),
+        }
+    }
+
+    /// Emit a diagnostic declaring that the object represented by `node` is not iterable
+    pub(super) fn add_not_iterable(&mut self, node: AnyNodeRef, not_iterable_ty: Type<'db>) {
+        self.add(
+            node,
+            "not-iterable",
+            format_args!(
+                "Object of type `{}` is not iterable",
+                not_iterable_ty.display(self.db)
+            ),
+        );
+    }
+
+    /// Emit a diagnostic declaring that the object represented by `node` is not iterable
+    /// because its `__iter__` method is possibly unbound.
+    pub(super) fn add_not_iterable_possibly_unbound(
+        &mut self,
+        node: AnyNodeRef,
+        element_ty: Type<'db>,
+    ) {
+        self.add(
+            node,
+            "not-iterable",
+            format_args!(
+                "Object of type `{}` is not iterable because its `__iter__` method is possibly unbound",
+                element_ty.display(self.db)
+            ),
+        );
+    }
+
+    /// Emit a diagnostic declaring that an index is out of bounds for a tuple.
+    pub(super) fn add_index_out_of_bounds(
+        &mut self,
+        kind: &'static str,
+        node: AnyNodeRef,
+        tuple_ty: Type<'db>,
+        length: usize,
+        index: i64,
+    ) {
+        self.add(
+            node,
+            "index-out-of-bounds",
+            format_args!(
+                "Index {index} is out of bounds for {kind} `{}` with length {length}",
+                tuple_ty.display(self.db)
+            ),
+        );
+    }
+
+    /// Emit a diagnostic declaring that a type does not support subscripting.
+    pub(super) fn add_non_subscriptable(
+        &mut self,
+        node: AnyNodeRef,
+        non_subscriptable_ty: Type<'db>,
+        method: &str,
+    ) {
+        self.add(
+            node,
+            "non-subscriptable",
+            format_args!(
+                "Cannot subscript object of type `{}` with no `{method}` method",
+                non_subscriptable_ty.display(self.db)
+            ),
+        );
+    }
+
+    pub(super) fn add_unresolved_module(
+        &mut self,
+        import_node: impl Into<AnyNodeRef<'db>>,
+        level: u32,
+        module: Option<&str>,
+    ) {
+        self.add(
+            import_node.into(),
+            "unresolved-import",
+            format_args!(
+                "Cannot resolve import `{}{}`",
+                ".".repeat(level as usize),
+                module.unwrap_or_default()
+            ),
+        );
+    }
+
+    pub(super) fn add_slice_step_size_zero(&mut self, node: AnyNodeRef) {
+        self.add(
+            node,
+            "zero-stepsize-in-slice",
+            format_args!("Slice step size can not be zero"),
+        );
+    }
+
+    pub(super) fn add_invalid_assignment(
+        &mut self,
+        node: AnyNodeRef,
+        declared_ty: Type<'db>,
+        assigned_ty: Type<'db>,
+    ) {
+        match declared_ty {
+            Type::ClassLiteral(ClassLiteralType { class }) => {
+                self.add(node, "invalid-assignment", format_args!(
+                        "Implicit shadowing of class `{}`; annotate to make it explicit if this is intentional",
+                        class.name(self.db)));
+            }
+            Type::FunctionLiteral(function) => {
+                self.add(node, "invalid-assignment", format_args!(
+                        "Implicit shadowing of function `{}`; annotate to make it explicit if this is intentional",
+                        function.name(self.db)));
+            }
+            _ => {
+                self.add(
+                    node,
+                    "invalid-assignment",
+                    format_args!(
+                        "Object of type `{}` is not assignable to `{}`",
+                        assigned_ty.display(self.db),
+                        declared_ty.display(self.db),
+                    ),
+                );
+            }
+        }
+    }
+
+    pub(super) fn add_possibly_unresolved_reference(&mut self, expr_name_node: &ast::ExprName) {
+        let ast::ExprName { id, .. } = expr_name_node;
+
+        self.add(
+            expr_name_node.into(),
+            "possibly-unresolved-reference",
+            format_args!("Name `{id}` used when possibly not defined"),
+        );
+    }
+
+    pub(super) fn add_unresolved_reference(&mut self, expr_name_node: &ast::ExprName) {
+        let ast::ExprName { id, .. } = expr_name_node;
+
+        self.add(
+            expr_name_node.into(),
+            "unresolved-reference",
+            format_args!("Name `{id}` used when not defined"),
+        );
+    }
+
+    /// Adds a new diagnostic.
+    ///
+    /// The diagnostic does not get added if the rule isn't enabled for this file.
+    pub(super) fn add(&mut self, node: AnyNodeRef, rule: &str, message: std::fmt::Arguments) {
+        if !self.db.is_file_open(self.file) {
+            return;
+        }
+
+        // TODO: Don't emit the diagnostic if:
+        // * The enclosing node contains any syntax errors
+        // * The rule is disabled for this file. We probably want to introduce a new query that
+        //   returns a rule selector for a given file that respects the package's settings,
+        //   any global pragma comments in the file, and any per-file-ignores.
+
+        self.diagnostics.push(TypeCheckDiagnostic {
+            file: self.file,
+            rule: rule.to_string(),
+            message: message.to_string(),
+            range: node.range(),
+        });
+    }
+
+    pub(super) fn extend(&mut self, diagnostics: &TypeCheckDiagnostics) {
+        self.diagnostics.extend(diagnostics);
+    }
+
+    pub(super) fn finish(mut self) -> TypeCheckDiagnostics {
+        self.diagnostics.shrink_to_fit();
+        self.diagnostics
+    }
+}

crates/red_knot_python_semantic/src/types/display.rs

@@ -6,7 +6,9 @@ use ruff_db::display::FormatterJoinExtension;
 use ruff_python_ast::str::Quote;
 use ruff_python_literal::escape::AsciiEscape;
 
-use crate::types::{IntersectionType, Type, UnionType};
+use crate::types::{
+    ClassLiteralType, InstanceType, IntersectionType, KnownClass, SubclassOfType, Type, UnionType,
+};
 use crate::Db;
 use rustc_hash::FxHashMap;
 
@@ -64,8 +66,11 @@ impl Display for DisplayRepresentation<'_> {
             Type::Any => f.write_str("Any"),
             Type::Never => f.write_str("Never"),
             Type::Unknown => f.write_str("Unknown"),
-            Type::Unbound => f.write_str("Unbound"),
-            Type::None => f.write_str("None"),
+            Type::Instance(InstanceType { class })
+                if class.is_known(self.db, KnownClass::NoneType) =>
+            {
+                f.write_str("None")
+            }
             // `[Type::Todo]`'s display should be explicit that is not a valid display of
             // any other type
             Type::Todo => f.write_str("@Todo"),
@@ -73,8 +78,12 @@ impl Display for DisplayRepresentation<'_> {
                 write!(f, "<module '{:?}'>", file.path(self.db))
             }
             // TODO functions and classes should display using a fully qualified name
-            Type::ClassLiteral(class) => f.write_str(class.name(self.db)),
-            Type::Instance(class) => f.write_str(class.name(self.db)),
+            Type::ClassLiteral(ClassLiteralType { class }) => f.write_str(class.name(self.db)),
+            Type::SubclassOf(SubclassOfType { class }) => {
+                write!(f, "type[{}]", class.name(self.db))
+            }
+            Type::Instance(InstanceType { class }) => f.write_str(class.name(self.db)),
+            Type::KnownInstance(known_instance) => f.write_str(known_instance.as_str()),
             Type::FunctionLiteral(function) => f.write_str(function.name(self.db)),
             Type::Union(union) => union.display(self.db).fmt(f),
             Type::Intersection(intersection) => intersection.display(self.db).fmt(f),
@@ -90,6 +99,28 @@ impl Display for DisplayRepresentation<'_> {
 
                 escape.bytes_repr().write(f)
             }
+            Type::SliceLiteral(slice) => {
+                f.write_str("slice[")?;
+                if let Some(start) = slice.start(self.db) {
+                    write!(f, "Literal[{start}]")?;
+                } else {
+                    f.write_str("None")?;
+                }
+
+                f.write_str(", ")?;
+
+                if let Some(stop) = slice.stop(self.db) {
+                    write!(f, "Literal[{stop}]")?;
+                } else {
+                    f.write_str("None")?;
+                }
+
+                if let Some(step) = slice.step(self.db) {
+                    write!(f, ", Literal[{step}]")?;
+                }
+
+                f.write_str("]")
+            }
             Type::Tuple(tuple) => {
                 f.write_str("tuple[")?;
                 let elements = tuple.elements(self.db);
@@ -301,7 +332,9 @@ mod tests {
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
 
     use crate::db::tests::TestDb;
-    use crate::types::{global_symbol_ty, BytesLiteralType, StringLiteralType, Type, UnionType};
+    use crate::types::{
+        global_symbol, BytesLiteralType, SliceLiteralType, StringLiteralType, Type, UnionType,
+    };
     use crate::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
 
     fn setup_db() -> TestDb {
@@ -346,18 +379,18 @@ mod tests {
         let union_elements = &[
             Type::Unknown,
             Type::IntLiteral(-1),
-            global_symbol_ty(&db, mod_file, "A"),
+            global_symbol(&db, mod_file, "A").expect_type(),
             Type::StringLiteral(StringLiteralType::new(&db, "A")),
             Type::BytesLiteral(BytesLiteralType::new(&db, [0u8].as_slice())),
             Type::BytesLiteral(BytesLiteralType::new(&db, [7u8].as_slice())),
             Type::IntLiteral(0),
             Type::IntLiteral(1),
             Type::StringLiteral(StringLiteralType::new(&db, "B")),
-            global_symbol_ty(&db, mod_file, "foo"),
-            global_symbol_ty(&db, mod_file, "bar"),
-            global_symbol_ty(&db, mod_file, "B"),
+            global_symbol(&db, mod_file, "foo").expect_type(),
+            global_symbol(&db, mod_file, "bar").expect_type(),
+            global_symbol(&db, mod_file, "B").expect_type(),
             Type::BooleanLiteral(true),
-            Type::None,
+            Type::none(&db),
         ];
         let union = UnionType::from_elements(&db, union_elements).expect_union();
         let display = format!("{}", union.display(&db));
@@ -376,4 +409,46 @@ mod tests {
         );
         Ok(())
     }
+
+    #[test]
+    fn test_slice_literal_display() {
+        let db = setup_db();
+
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, None, None, None))
+                .display(&db)
+                .to_string(),
+            "slice[None, None]"
+        );
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, Some(1), None, None))
+                .display(&db)
+                .to_string(),
+            "slice[Literal[1], None]"
+        );
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, None, Some(2), None))
+                .display(&db)
+                .to_string(),
+            "slice[None, Literal[2]]"
+        );
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, Some(1), Some(5), None))
+                .display(&db)
+                .to_string(),
+            "slice[Literal[1], Literal[5]]"
+        );
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, Some(1), Some(5), Some(2)))
+                .display(&db)
+                .to_string(),
+            "slice[Literal[1], Literal[5], Literal[2]]"
+        );
+        assert_eq!(
+            Type::SliceLiteral(SliceLiteralType::new(&db, None, None, Some(2)))
+                .display(&db)
+                .to_string(),
+            "slice[None, None, Literal[2]]"
+        );
+    }
 }

crates/red_knot_python_semantic/src/types/mro.rs

@@ -0,0 +1,505 @@
+use std::collections::VecDeque;
+use std::ops::Deref;
+
+use indexmap::IndexSet;
+use itertools::Either;
+use rustc_hash::FxHashSet;
+
+use super::{Class, ClassLiteralType, KnownClass, KnownInstanceType, Type};
+use crate::Db;
+
+/// The inferred method resolution order of a given class.
+///
+/// See [`Class::iter_mro`] for more details.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub(super) struct Mro<'db>(Box<[ClassBase<'db>]>);
+
+impl<'db> Mro<'db> {
+    /// Attempt to resolve the MRO of a given class
+    ///
+    /// In the event that a possible list of bases would (or could) lead to a
+    /// `TypeError` being raised at runtime due to an unresolvable MRO, we infer
+    /// the MRO of the class as being `[<the class in question>, Unknown, object]`.
+    /// This seems most likely to reduce the possibility of cascading errors
+    /// elsewhere.
+    ///
+    /// (We emit a diagnostic warning about the runtime `TypeError` in
+    /// [`super::infer::TypeInferenceBuilder::infer_region_scope`].)
+    pub(super) fn of_class(db: &'db dyn Db, class: Class<'db>) -> Result<Self, MroError<'db>> {
+        Self::of_class_impl(db, class).map_err(|error_kind| {
+            let fallback_mro = Self::from([
+                ClassBase::Class(class),
+                ClassBase::Unknown,
+                ClassBase::object(db),
+            ]);
+            MroError {
+                kind: error_kind,
+                fallback_mro,
+            }
+        })
+    }
+
+    fn of_class_impl(db: &'db dyn Db, class: Class<'db>) -> Result<Self, MroErrorKind<'db>> {
+        let class_bases = class.explicit_bases(db);
+
+        match class_bases {
+            // `builtins.object` is the special case:
+            // the only class in Python that has an MRO with length <2
+            [] if class.is_known(db, KnownClass::Object) => {
+                Ok(Self::from([ClassBase::Class(class)]))
+            }
+
+            // All other classes in Python have an MRO with length >=2.
+            // Even if a class has no explicit base classes,
+            // it will implicitly inherit from `object` at runtime;
+            // `object` will appear in the class's `__bases__` list and `__mro__`:
+            //
+            // ```pycon
+            // >>> class Foo: ...
+            // ...
+            // >>> Foo.__bases__
+            // (<class 'object'>,)
+            // >>> Foo.__mro__
+            // (<class '__main__.Foo'>, <class 'object'>)
+            // ```
+            [] => Ok(Self::from([ClassBase::Class(class), ClassBase::object(db)])),
+
+            // Fast path for a class that has only a single explicit base.
+            //
+            // This *could* theoretically be handled by the final branch below,
+            // but it's a common case (i.e., worth optimizing for),
+            // and the `c3_merge` function requires lots of allocations.
+            [single_base] => {
+                let single_base = ClassBase::try_from_ty(*single_base).ok_or(*single_base);
+                single_base.map_or_else(
+                    |invalid_base_ty| {
+                        let bases_info = Box::from([(0, invalid_base_ty)]);
+                        Err(MroErrorKind::InvalidBases(bases_info))
+                    },
+                    |single_base| {
+                        if let ClassBase::Class(class_base) = single_base {
+                            if class_is_cyclically_defined(db, class_base) {
+                                return Err(MroErrorKind::CyclicClassDefinition);
+                            }
+                        }
+                        let mro = std::iter::once(ClassBase::Class(class))
+                            .chain(single_base.mro(db))
+                            .collect();
+                        Ok(mro)
+                    },
+                )
+            }
+
+            // The class has multiple explicit bases.
+            //
+            // We'll fallback to a full implementation of the C3-merge algorithm to determine
+            // what MRO Python will give this class at runtime
+            // (if an MRO is indeed resolvable at all!)
+            multiple_bases => {
+                if class_is_cyclically_defined(db, class) {
+                    return Err(MroErrorKind::CyclicClassDefinition);
+                }
+
+                let mut valid_bases = vec![];
+                let mut invalid_bases = vec![];
+
+                for (i, base) in multiple_bases.iter().enumerate() {
+                    match ClassBase::try_from_ty(*base).ok_or(*base) {
+                        Ok(valid_base) => valid_bases.push(valid_base),
+                        Err(invalid_base) => invalid_bases.push((i, invalid_base)),
+                    }
+                }
+
+                if !invalid_bases.is_empty() {
+                    return Err(MroErrorKind::InvalidBases(invalid_bases.into_boxed_slice()));
+                }
+
+                let mut seqs = vec![VecDeque::from([ClassBase::Class(class)])];
+                for base in &valid_bases {
+                    seqs.push(base.mro(db).collect());
+                }
+                seqs.push(valid_bases.iter().copied().collect());
+
+                c3_merge(seqs).ok_or_else(|| {
+                    let mut seen_bases = FxHashSet::default();
+                    let mut duplicate_bases = vec![];
+                    for (index, base) in valid_bases
+                        .iter()
+                        .enumerate()
+                        .filter_map(|(index, base)| Some((index, base.into_class_literal_type()?)))
+                    {
+                        if !seen_bases.insert(base) {
+                            duplicate_bases.push((index, base));
+                        }
+                    }
+
+                    if duplicate_bases.is_empty() {
+                        MroErrorKind::UnresolvableMro {
+                            bases_list: valid_bases.into_boxed_slice(),
+                        }
+                    } else {
+                        MroErrorKind::DuplicateBases(duplicate_bases.into_boxed_slice())
+                    }
+                })
+            }
+        }
+    }
+}
+
+impl<'db, const N: usize> From<[ClassBase<'db>; N]> for Mro<'db> {
+    fn from(value: [ClassBase<'db>; N]) -> Self {
+        Self(Box::from(value))
+    }
+}
+
+impl<'db> From<Vec<ClassBase<'db>>> for Mro<'db> {
+    fn from(value: Vec<ClassBase<'db>>) -> Self {
+        Self(value.into_boxed_slice())
+    }
+}
+
+impl<'db> Deref for Mro<'db> {
+    type Target = [ClassBase<'db>];
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'db> FromIterator<ClassBase<'db>> for Mro<'db> {
+    fn from_iter<T: IntoIterator<Item = ClassBase<'db>>>(iter: T) -> Self {
+        Self(iter.into_iter().collect())
+    }
+}
+
+/// Iterator that yields elements of a class's MRO.
+///
+/// We avoid materialising the *full* MRO unless it is actually necessary:
+/// - Materialising the full MRO is expensive
+/// - We need to do it for every class in the code that we're checking, as we need to make sure
+///   that there are no class definitions in the code we're checking that would cause an
+///   exception to be raised at runtime. But the same does *not* necessarily apply for every class
+///   in third-party and stdlib dependencies: we never emit diagnostics about non-first-party code.
+/// - However, we *do* need to resolve attribute accesses on classes/instances from
+///   third-party and stdlib dependencies. That requires iterating over the MRO of third-party/stdlib
+///   classes, but not necessarily the *whole* MRO: often just the first element is enough.
+///   Luckily we know that for any class `X`, the first element of `X`'s MRO will always be `X` itself.
+///   We can therefore avoid resolving the full MRO for many third-party/stdlib classes while still
+///   being faithful to the runtime semantics.
+///
+/// Even for first-party code, where we will have to resolve the MRO for every class we encounter,
+/// loading the cached MRO comes with a certain amount of overhead, so it's best to avoid calling the
+/// Salsa-tracked [`Class::try_mro`] method unless it's absolutely necessary.
+pub(super) struct MroIterator<'db> {
+    db: &'db dyn Db,
+
+    /// The class whose MRO we're iterating over
+    class: Class<'db>,
+
+    /// Whether or not we've already yielded the first element of the MRO
+    first_element_yielded: bool,
+
+    /// Iterator over all elements of the MRO except the first.
+    ///
+    /// The full MRO is expensive to materialize, so this field is `None`
+    /// unless we actually *need* to iterate past the first element of the MRO,
+    /// at which point it is lazily materialized.
+    subsequent_elements: Option<std::slice::Iter<'db, ClassBase<'db>>>,
+}
+
+impl<'db> MroIterator<'db> {
+    pub(super) fn new(db: &'db dyn Db, class: Class<'db>) -> Self {
+        Self {
+            db,
+            class,
+            first_element_yielded: false,
+            subsequent_elements: None,
+        }
+    }
+
+    /// Materialize the full MRO of the class.
+    /// Return an iterator over that MRO which skips the first element of the MRO.
+    fn full_mro_except_first_element(&mut self) -> impl Iterator<Item = ClassBase<'db>> + '_ {
+        self.subsequent_elements
+            .get_or_insert_with(|| {
+                let mut full_mro_iter = match self.class.try_mro(self.db) {
+                    Ok(mro) => mro.iter(),
+                    Err(error) => error.fallback_mro().iter(),
+                };
+                full_mro_iter.next();
+                full_mro_iter
+            })
+            .copied()
+    }
+}
+
+impl<'db> Iterator for MroIterator<'db> {
+    type Item = ClassBase<'db>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if !self.first_element_yielded {
+            self.first_element_yielded = true;
+            return Some(ClassBase::Class(self.class));
+        }
+        self.full_mro_except_first_element().next()
+    }
+}
+
+impl std::iter::FusedIterator for MroIterator<'_> {}
+
+#[derive(Debug, PartialEq, Eq)]
+pub(super) struct MroError<'db> {
+    kind: MroErrorKind<'db>,
+    fallback_mro: Mro<'db>,
+}
+
+impl<'db> MroError<'db> {
+    /// Return an [`MroErrorKind`] variant describing why we could not resolve the MRO for this class.
+    pub(super) fn reason(&self) -> &MroErrorKind<'db> {
+        &self.kind
+    }
+
+    /// Return the fallback MRO we should infer for this class during type inference
+    /// (since accurate resolution of its "true" MRO was impossible)
+    pub(super) fn fallback_mro(&self) -> &Mro<'db> {
+        &self.fallback_mro
+    }
+}
+
+/// Possible ways in which attempting to resolve the MRO of a class might fail.
+#[derive(Debug, PartialEq, Eq)]
+pub(super) enum MroErrorKind<'db> {
+    /// The class inherits from one or more invalid bases.
+    ///
+    /// To avoid excessive complexity in our implementation,
+    /// we only permit classes to inherit from class-literal types,
+    /// `Todo`, `Unknown` or `Any`. Anything else results in us
+    /// emitting a diagnostic.
+    ///
+    /// This variant records the indices and types of class bases
+    /// that we deem to be invalid. The indices are the indices of nodes
+    /// in the bases list of the class's [`StmtClassDef`](ruff_python_ast::StmtClassDef) node.
+    /// Each index is the index of a node representing an invalid base.
+    InvalidBases(Box<[(usize, Type<'db>)]>),
+
+    /// The class inherits from itself!
+    ///
+    /// This is very unlikely to happen in working real-world code,
+    /// but it's important to explicitly account for it.
+    /// If we don't, there's a possibility of an infinite loop and a panic.
+    CyclicClassDefinition,
+
+    /// The class has one or more duplicate bases.
+    ///
+    /// This variant records the indices and [`Class`]es
+    /// of the duplicate bases. The indices are the indices of nodes
+    /// in the bases list of the class's [`StmtClassDef`](ruff_python_ast::StmtClassDef) node.
+    /// Each index is the index of a node representing a duplicate base.
+    DuplicateBases(Box<[(usize, Class<'db>)]>),
+
+    /// The MRO is otherwise unresolvable through the C3-merge algorithm.
+    ///
+    /// See [`c3_merge`] for more details.
+    UnresolvableMro { bases_list: Box<[ClassBase<'db>]> },
+}
+
+/// Enumeration of the possible kinds of types we allow in class bases.
+///
+/// This is much more limited than the [`Type`] enum:
+/// all types that would be invalid to have as a class base are
+/// transformed into [`ClassBase::Unknown`]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub(super) enum ClassBase<'db> {
+    Any,
+    Unknown,
+    Todo,
+    Class(Class<'db>),
+}
+
+impl<'db> ClassBase<'db> {
+    pub(super) fn display(self, db: &'db dyn Db) -> impl std::fmt::Display + 'db {
+        struct Display<'db> {
+            base: ClassBase<'db>,
+            db: &'db dyn Db,
+        }
+
+        impl std::fmt::Display for Display<'_> {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                match self.base {
+                    ClassBase::Any => f.write_str("Any"),
+                    ClassBase::Todo => f.write_str("Todo"),
+                    ClassBase::Unknown => f.write_str("Unknown"),
+                    ClassBase::Class(class) => write!(f, "<class '{}'>", class.name(self.db)),
+                }
+            }
+        }
+
+        Display { base: self, db }
+    }
+
+    #[cfg(test)]
+    #[track_caller]
+    pub(super) fn expect_class_base(self) -> Class<'db> {
+        match self {
+            ClassBase::Class(class) => class,
+            _ => panic!("Expected a `ClassBase::Class()` variant"),
+        }
+    }
+
+    /// Return a `ClassBase` representing the class `builtins.object`
+    fn object(db: &'db dyn Db) -> Self {
+        KnownClass::Object
+            .to_class(db)
+            .into_class_literal()
+            .map_or(Self::Unknown, |ClassLiteralType { class }| {
+                Self::Class(class)
+            })
+    }
+
+    /// Attempt to resolve `ty` into a `ClassBase`.
+    ///
+    /// Return `None` if `ty` is not an acceptable type for a class base.
+    fn try_from_ty(ty: Type<'db>) -> Option<Self> {
+        match ty {
+            Type::Any => Some(Self::Any),
+            Type::Unknown => Some(Self::Unknown),
+            Type::Todo => Some(Self::Todo),
+            Type::ClassLiteral(ClassLiteralType { class }) => Some(Self::Class(class)),
+            Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
+            Type::Intersection(_) => None, // TODO -- probably incorrect?
+            Type::Instance(_) => None, // TODO -- handle `__mro_entries__`?
+            Type::Never
+            | Type::BooleanLiteral(_)
+            | Type::FunctionLiteral(_)
+            | Type::BytesLiteral(_)
+            | Type::IntLiteral(_)
+            | Type::StringLiteral(_)
+            | Type::LiteralString
+            | Type::Tuple(_)
+            | Type::SliceLiteral(_)
+            | Type::ModuleLiteral(_)
+            | Type::SubclassOf(_) => None,
+            Type::KnownInstance(known_instance) => match known_instance {
+                KnownInstanceType::Literal => None,
+            },
+        }
+    }
+
+    fn into_class_literal_type(self) -> Option<Class<'db>> {
+        match self {
+            Self::Class(class) => Some(class),
+            _ => None,
+        }
+    }
+
+    /// Iterate over the MRO of this base
+    fn mro(
+        self,
+        db: &'db dyn Db,
+    ) -> Either<impl Iterator<Item = ClassBase<'db>>, impl Iterator<Item = ClassBase<'db>>> {
+        match self {
+            ClassBase::Any => Either::Left([ClassBase::Any, ClassBase::object(db)].into_iter()),
+            ClassBase::Unknown => {
+                Either::Left([ClassBase::Unknown, ClassBase::object(db)].into_iter())
+            }
+            ClassBase::Todo => Either::Left([ClassBase::Todo, ClassBase::object(db)].into_iter()),
+            ClassBase::Class(class) => Either::Right(class.iter_mro(db)),
+        }
+    }
+}
+
+impl<'db> From<ClassBase<'db>> for Type<'db> {
+    fn from(value: ClassBase<'db>) -> Self {
+        match value {
+            ClassBase::Any => Type::Any,
+            ClassBase::Todo => Type::Todo,
+            ClassBase::Unknown => Type::Unknown,
+            ClassBase::Class(class) => Type::ClassLiteral(ClassLiteralType { class }),
+        }
+    }
+}
+
+/// Implementation of the [C3-merge algorithm] for calculating a Python class's
+/// [method resolution order].
+///
+/// [C3-merge algorithm]: https://docs.python.org/3/howto/mro.html#python-2-3-mro
+/// [method resolution order]: https://docs.python.org/3/glossary.html#term-method-resolution-order
+fn c3_merge(mut sequences: Vec<VecDeque<ClassBase>>) -> Option<Mro> {
+    // Most MROs aren't that long...
+    let mut mro = Vec::with_capacity(8);
+
+    loop {
+        sequences.retain(|sequence| !sequence.is_empty());
+
+        if sequences.is_empty() {
+            return Some(Mro::from(mro));
+        }
+
+        // If the candidate exists "deeper down" in the inheritance hierarchy,
+        // we should refrain from adding it to the MRO for now. Add the first candidate
+        // for which this does not hold true. If this holds true for all candidates,
+        // return `None`; it will be impossible to find a consistent MRO for the class
+        // with the given bases.
+        let mro_entry = sequences.iter().find_map(|outer_sequence| {
+            let candidate = outer_sequence[0];
+
+            let not_head = sequences
+                .iter()
+                .all(|sequence| sequence.iter().skip(1).all(|base| base != &candidate));
+
+            not_head.then_some(candidate)
+        })?;
+
+        mro.push(mro_entry);
+
+        // Make sure we don't try to add the candidate to the MRO twice:
+        for sequence in &mut sequences {
+            if sequence[0] == mro_entry {
+                sequence.pop_front();
+            }
+        }
+    }
+}
+
+/// Return `true` if this class appears to be a cyclic definition,
+/// i.e., it inherits either directly or indirectly from itself.
+///
+/// A class definition like this will fail at runtime,
+/// but we must be resilient to it or we could panic.
+fn class_is_cyclically_defined(db: &dyn Db, class: Class) -> bool {
+    fn is_cyclically_defined_recursive<'db>(
+        db: &'db dyn Db,
+        class: Class<'db>,
+        classes_to_watch: &mut IndexSet<Class<'db>>,
+    ) -> bool {
+        if !classes_to_watch.insert(class) {
+            return true;
+        }
+        for explicit_base_class in class
+            .explicit_bases(db)
+            .iter()
+            .copied()
+            .filter_map(Type::into_class_literal)
+            .map(|ClassLiteralType { class }| class)
+        {
+            // Each base must be considered in isolation.
+            // This is due to the fact that if a class uses multiple inheritance,
+            // there could easily be a situation where two bases have the same class in their MROs;
+            // that isn't enough to constitute the class being cyclically defined.
+            let classes_to_watch_len = classes_to_watch.len();
+            if is_cyclically_defined_recursive(db, explicit_base_class, classes_to_watch) {
+                return true;
+            }
+            classes_to_watch.truncate(classes_to_watch_len);
+        }
+        false
+    }
+
+    class
+        .explicit_bases(db)
+        .iter()
+        .copied()
+        .filter_map(Type::into_class_literal)
+        .map(|ClassLiteralType { class }| class)
+        .any(|base_class| is_cyclically_defined_recursive(db, base_class, &mut IndexSet::default()))
+}

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -1,16 +1,19 @@
 use crate::semantic_index::ast_ids::HasScopedAstId;
-use crate::semantic_index::constraint::{Constraint, PatternConstraint};
+use crate::semantic_index::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{ScopeId, ScopedSymbolId, SymbolTable};
 use crate::semantic_index::symbol_table;
 use crate::types::{
-    infer_expression_types, IntersectionBuilder, KnownFunction, Type, UnionBuilder,
+    infer_expression_types, ClassLiteralType, InstanceType, IntersectionBuilder, KnownClass,
+    KnownConstraintFunction, KnownFunction, Truthiness, Type, UnionBuilder,
 };
 use crate::Db;
 use itertools::Itertools;
 use ruff_python_ast as ast;
+use ruff_python_ast::{BoolOp, ExprBoolOp};
 use rustc_hash::FxHashMap;
+use std::collections::hash_map::Entry;
 use std::sync::Arc;
 
 /// Return the type constraint that `test` (if true) would place on `definition`, if any.
@@ -34,15 +37,20 @@ pub(crate) fn narrowing_constraint<'db>(
     constraint: Constraint<'db>,
     definition: Definition<'db>,
 ) -> Option<Type<'db>> {
-    match constraint {
-        Constraint::Expression(expression) => {
-            all_narrowing_constraints_for_expression(db, expression)
-                .get(&definition.symbol(db))
-                .copied()
+    let constraints = match constraint.node {
+        ConstraintNode::Expression(expression) => {
+            if constraint.is_positive {
+                all_narrowing_constraints_for_expression(db, expression)
+            } else {
+                all_negative_narrowing_constraints_for_expression(db, expression)
+            }
         }
-        Constraint::Pattern(pattern) => all_narrowing_constraints_for_pattern(db, pattern)
-            .get(&definition.symbol(db))
-            .copied(),
+        ConstraintNode::Pattern(pattern) => all_narrowing_constraints_for_pattern(db, pattern),
+    };
+    if let Some(constraints) = constraints {
+        constraints.get(&definition.symbol(db)).copied()
+    } else {
+        None
     }
 }
 
@@ -50,106 +58,184 @@ pub(crate) fn narrowing_constraint<'db>(
 fn all_narrowing_constraints_for_pattern<'db>(
     db: &'db dyn Db,
     pattern: PatternConstraint<'db>,
-) -> NarrowingConstraints<'db> {
-    NarrowingConstraintsBuilder::new(db, Constraint::Pattern(pattern)).finish()
+) -> Option<NarrowingConstraints<'db>> {
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Pattern(pattern), true).finish()
 }
 
 #[salsa::tracked(return_ref)]
 fn all_narrowing_constraints_for_expression<'db>(
     db: &'db dyn Db,
     expression: Expression<'db>,
-) -> NarrowingConstraints<'db> {
-    NarrowingConstraintsBuilder::new(db, Constraint::Expression(expression)).finish()
+) -> Option<NarrowingConstraints<'db>> {
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Expression(expression), true).finish()
 }
 
-/// Generate a constraint from the *type* of the second argument of an `isinstance` call.
+#[salsa::tracked(return_ref)]
+fn all_negative_narrowing_constraints_for_expression<'db>(
+    db: &'db dyn Db,
+    expression: Expression<'db>,
+) -> Option<NarrowingConstraints<'db>> {
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Expression(expression), false).finish()
+}
+
+/// Generate a constraint from the type of a `classinfo` argument to `isinstance` or `issubclass`.
 ///
-/// Example: for `isinstance(…, str)`, we would infer `Type::ClassLiteral(str)` from the
-/// second argument, but we need to generate a `Type::Instance(str)` constraint that can
-/// be used to narrow down the type of the first argument.
-fn generate_isinstance_constraint<'db>(
+/// The `classinfo` argument can be a class literal, a tuple of (tuples of) class literals. PEP 604
+/// union types are not yet supported. Returns `None` if the `classinfo` argument has a wrong type.
+fn generate_classinfo_constraint<'db, F>(
     db: &'db dyn Db,
     classinfo: &Type<'db>,
-) -> Option<Type<'db>> {
+    to_constraint: F,
+) -> Option<Type<'db>>
+where
+    F: Fn(ClassLiteralType<'db>) -> Type<'db> + Copy,
+{
     match classinfo {
-        Type::ClassLiteral(class) => Some(Type::Instance(*class)),
         Type::Tuple(tuple) => {
             let mut builder = UnionBuilder::new(db);
             for element in tuple.elements(db) {
-                builder = builder.add(generate_isinstance_constraint(db, element)?);
+                builder = builder.add(generate_classinfo_constraint(db, element, to_constraint)?);
             }
             Some(builder.build())
         }
+        Type::ClassLiteral(class_literal_type) => Some(to_constraint(*class_literal_type)),
         _ => None,
     }
 }
 
 type NarrowingConstraints<'db> = FxHashMap<ScopedSymbolId, Type<'db>>;
 
+fn merge_constraints_and<'db>(
+    into: &mut NarrowingConstraints<'db>,
+    from: NarrowingConstraints<'db>,
+    db: &'db dyn Db,
+) {
+    for (key, value) in from {
+        match into.entry(key) {
+            Entry::Occupied(mut entry) => {
+                *entry.get_mut() = IntersectionBuilder::new(db)
+                    .add_positive(*entry.get())
+                    .add_positive(value)
+                    .build();
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(value);
+            }
+        }
+    }
+}
+
+fn merge_constraints_or<'db>(
+    into: &mut NarrowingConstraints<'db>,
+    from: &NarrowingConstraints<'db>,
+    db: &'db dyn Db,
+) {
+    for (key, value) in from {
+        match into.entry(*key) {
+            Entry::Occupied(mut entry) => {
+                *entry.get_mut() = UnionBuilder::new(db).add(*entry.get()).add(*value).build();
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(KnownClass::Object.to_instance(db));
+            }
+        }
+    }
+    for (key, value) in into.iter_mut() {
+        if !from.contains_key(key) {
+            *value = KnownClass::Object.to_instance(db);
+        }
+    }
+}
+
 struct NarrowingConstraintsBuilder<'db> {
     db: &'db dyn Db,
-    constraint: Constraint<'db>,
-    constraints: NarrowingConstraints<'db>,
+    constraint: ConstraintNode<'db>,
+    is_positive: bool,
 }
 
 impl<'db> NarrowingConstraintsBuilder<'db> {
-    fn new(db: &'db dyn Db, constraint: Constraint<'db>) -> Self {
+    fn new(db: &'db dyn Db, constraint: ConstraintNode<'db>, is_positive: bool) -> Self {
         Self {
             db,
             constraint,
-            constraints: NarrowingConstraints::default(),
+            is_positive,
         }
     }
 
-    fn finish(mut self) -> NarrowingConstraints<'db> {
-        match self.constraint {
-            Constraint::Expression(expression) => self.evaluate_expression_constraint(expression),
-            Constraint::Pattern(pattern) => self.evaluate_pattern_constraint(pattern),
+    fn finish(mut self) -> Option<NarrowingConstraints<'db>> {
+        let constraints: Option<NarrowingConstraints<'db>> = match self.constraint {
+            ConstraintNode::Expression(expression) => {
+                self.evaluate_expression_constraint(expression, self.is_positive)
+            }
+            ConstraintNode::Pattern(pattern) => self.evaluate_pattern_constraint(pattern),
+        };
+        if let Some(mut constraints) = constraints {
+            constraints.shrink_to_fit();
+            Some(constraints)
+        } else {
+            None
         }
-
-        self.constraints.shrink_to_fit();
-        self.constraints
     }
 
-    fn evaluate_expression_constraint(&mut self, expression: Expression<'db>) {
-        match expression.node_ref(self.db).node() {
+    fn evaluate_expression_constraint(
+        &mut self,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) -> Option<NarrowingConstraints<'db>> {
+        let expression_node = expression.node_ref(self.db).node();
+        self.evaluate_expression_node_constraint(expression_node, expression, is_positive)
+    }
+
+    fn evaluate_expression_node_constraint(
+        &mut self,
+        expression_node: &ruff_python_ast::Expr,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) -> Option<NarrowingConstraints<'db>> {
+        match expression_node {
             ast::Expr::Compare(expr_compare) => {
-                self.add_expr_compare(expr_compare, expression);
+                self.evaluate_expr_compare(expr_compare, expression, is_positive)
             }
             ast::Expr::Call(expr_call) => {
-                self.add_expr_call(expr_call, expression);
+                self.evaluate_expr_call(expr_call, expression, is_positive)
             }
-            _ => {} // TODO other test expression kinds
+            ast::Expr::UnaryOp(unary_op) if unary_op.op == ast::UnaryOp::Not => self
+                .evaluate_expression_node_constraint(&unary_op.operand, expression, !is_positive),
+            ast::Expr::BoolOp(bool_op) => self.evaluate_bool_op(bool_op, expression, is_positive),
+            _ => None, // TODO other test expression kinds
         }
     }
 
-    fn evaluate_pattern_constraint(&mut self, pattern: PatternConstraint<'db>) {
+    fn evaluate_pattern_constraint(
+        &mut self,
+        pattern: PatternConstraint<'db>,
+    ) -> Option<NarrowingConstraints<'db>> {
         let subject = pattern.subject(self.db);
 
         match pattern.pattern(self.db).node() {
             ast::Pattern::MatchValue(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchSingleton(singleton_pattern) => {
-                self.add_match_pattern_singleton(subject, singleton_pattern);
+                self.evaluate_match_pattern_singleton(subject, singleton_pattern)
             }
             ast::Pattern::MatchSequence(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchMapping(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchClass(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchStar(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchAs(_) => {
-                // TODO
+                None // TODO
             }
             ast::Pattern::MatchOr(_) => {
-                // TODO
+                None // TODO
             }
         }
     }
@@ -160,12 +246,17 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
 
     fn scope(&self) -> ScopeId<'db> {
         match self.constraint {
-            Constraint::Expression(expression) => expression.scope(self.db),
-            Constraint::Pattern(pattern) => pattern.scope(self.db),
+            ConstraintNode::Expression(expression) => expression.scope(self.db),
+            ConstraintNode::Pattern(pattern) => pattern.scope(self.db),
         }
     }
 
-    fn add_expr_compare(&mut self, expr_compare: &ast::ExprCompare, expression: Expression<'db>) {
+    fn evaluate_expr_compare(
+        &mut self,
+        expr_compare: &ast::ExprCompare,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) -> Option<NarrowingConstraints<'db>> {
         let ast::ExprCompare {
             range: _,
             left,
@@ -175,7 +266,14 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         if !left.is_name_expr() && comparators.iter().all(|c| !c.is_name_expr()) {
             // If none of the comparators are name expressions,
             // we have no symbol to narrow down the type of.
-            return;
+            return None;
+        }
+        if !is_positive && comparators.len() > 1 {
+            // We can't negate a constraint made by a multi-comparator expression, since we can't
+            // know which comparison part is the one being negated.
+            // For example, the negation of  `x is 1 is y is 2`, would be `(x is not 1) or (y is not 1) or (y is not 2)`
+            // and that requires cross-symbol constraints, which we don't support yet.
+            return None;
         }
         let scope = self.scope();
         let inference = infer_expression_types(self.db, expression);
@@ -183,6 +281,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         let comparator_tuples = std::iter::once(&**left)
             .chain(comparators)
             .tuple_windows::<(&ruff_python_ast::Expr, &ruff_python_ast::Expr)>();
+        let mut constraints = NarrowingConstraints::default();
         for (op, (left, right)) in std::iter::zip(&**ops, comparator_tuples) {
             if let ast::Expr::Name(ast::ExprName {
                 range: _,
@@ -192,27 +291,28 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             {
                 // SAFETY: we should always have a symbol for every Name node.
                 let symbol = self.symbols().symbol_id_by_name(id).unwrap();
-                let comp_ty = inference.expression_ty(right.scoped_ast_id(self.db, scope));
-                match op {
+                let rhs_ty = inference.expression_ty(right.scoped_ast_id(self.db, scope));
+
+                match if is_positive { *op } else { op.negate() } {
                     ast::CmpOp::IsNot => {
-                        if comp_ty.is_singleton() {
+                        if rhs_ty.is_singleton(self.db) {
                             let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(comp_ty)
+                                .add_negative(rhs_ty)
                                 .build();
-                            self.constraints.insert(symbol, ty);
+                            constraints.insert(symbol, ty);
                         } else {
                             // Non-singletons cannot be safely narrowed using `is not`
                         }
                     }
                     ast::CmpOp::Is => {
-                        self.constraints.insert(symbol, comp_ty);
+                        constraints.insert(symbol, rhs_ty);
                     }
                     ast::CmpOp::NotEq => {
-                        if comp_ty.is_single_valued(self.db) {
+                        if rhs_ty.is_single_valued(self.db) {
                             let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(comp_ty)
+                                .add_negative(rhs_ty)
                                 .build();
-                            self.constraints.insert(symbol, ty);
+                            constraints.insert(symbol, ty);
                         }
                     }
                     _ => {
@@ -221,50 +321,137 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                 }
             }
         }
+        Some(constraints)
     }
 
-    fn add_expr_call(&mut self, expr_call: &ast::ExprCall, expression: Expression<'db>) {
+    fn evaluate_expr_call(
+        &mut self,
+        expr_call: &ast::ExprCall,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) -> Option<NarrowingConstraints<'db>> {
         let scope = self.scope();
         let inference = infer_expression_types(self.db, expression);
 
-        if let Some(func_type) = inference
+        // TODO: add support for PEP 604 union types on the right hand side of `isinstance`
+        // and `issubclass`, for example `isinstance(x, str | (int | float))`.
+        match inference
             .expression_ty(expr_call.func.scoped_ast_id(self.db, scope))
-            .into_function_literal_type()
+            .into_function_literal()
+            .and_then(|f| f.known(self.db))
+            .and_then(KnownFunction::constraint_function)
         {
-            if func_type.is_known(self.db, KnownFunction::IsInstance)
-                && expr_call.arguments.keywords.is_empty()
-            {
-                if let [ast::Expr::Name(ast::ExprName { id, .. }), rhs] = &*expr_call.arguments.args
+            Some(function) if expr_call.arguments.keywords.is_empty() => {
+                if let [ast::Expr::Name(ast::ExprName { id, .. }), class_info] =
+                    &*expr_call.arguments.args
                 {
                     let symbol = self.symbols().symbol_id_by_name(id).unwrap();
 
-                    let rhs_type = inference.expression_ty(rhs.scoped_ast_id(self.db, scope));
+                    let class_info_ty =
+                        inference.expression_ty(class_info.scoped_ast_id(self.db, scope));
 
-                    // TODO: add support for PEP 604 union types on the right hand side:
-                    // isinstance(x, str | (int | float))
-                    if let Some(constraint) = generate_isinstance_constraint(self.db, &rhs_type) {
-                        self.constraints.insert(symbol, constraint);
-                    }
+                    let to_constraint = match function {
+                        KnownConstraintFunction::IsInstance => {
+                            |class_literal: ClassLiteralType<'db>| {
+                                Type::Instance(InstanceType {
+                                    class: class_literal.class,
+                                })
+                            }
+                        }
+                        KnownConstraintFunction::IsSubclass => {
+                            |class_literal: ClassLiteralType<'db>| {
+                                Type::SubclassOf(class_literal.to_subclass_of_type())
+                            }
+                        }
+                    };
+
+                    generate_classinfo_constraint(self.db, &class_info_ty, to_constraint).map(
+                        |constraint| {
+                            let mut constraints = NarrowingConstraints::default();
+                            constraints.insert(symbol, constraint.negate_if(self.db, !is_positive));
+                            constraints
+                        },
+                    )
+                } else {
+                    None
                 }
             }
+            _ => None,
         }
     }
 
-    fn add_match_pattern_singleton(
+    fn evaluate_match_pattern_singleton(
         &mut self,
         subject: &ast::Expr,
         pattern: &ast::PatternMatchSingleton,
-    ) {
+    ) -> Option<NarrowingConstraints<'db>> {
         if let Some(ast::ExprName { id, .. }) = subject.as_name_expr() {
             // SAFETY: we should always have a symbol for every Name node.
             let symbol = self.symbols().symbol_id_by_name(id).unwrap();
 
             let ty = match pattern.value {
-                ast::Singleton::None => Type::None,
+                ast::Singleton::None => Type::none(self.db),
                 ast::Singleton::True => Type::BooleanLiteral(true),
                 ast::Singleton::False => Type::BooleanLiteral(false),
             };
-            self.constraints.insert(symbol, ty);
+            let mut constraints = NarrowingConstraints::default();
+            constraints.insert(symbol, ty);
+            Some(constraints)
+        } else {
+            None
+        }
+    }
+
+    fn evaluate_bool_op(
+        &mut self,
+        expr_bool_op: &ExprBoolOp,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) -> Option<NarrowingConstraints<'db>> {
+        let inference = infer_expression_types(self.db, expression);
+        let scope = self.scope();
+        let mut sub_constraints = expr_bool_op
+            .values
+            .iter()
+            // filter our arms with statically known truthiness
+            .filter(|expr| {
+                inference
+                    .expression_ty(expr.scoped_ast_id(self.db, scope))
+                    .bool(self.db)
+                    != match expr_bool_op.op {
+                        BoolOp::And => Truthiness::AlwaysTrue,
+                        BoolOp::Or => Truthiness::AlwaysFalse,
+                    }
+            })
+            .map(|sub_expr| {
+                self.evaluate_expression_node_constraint(sub_expr, expression, is_positive)
+            })
+            .collect::<Vec<_>>();
+        match (expr_bool_op.op, is_positive) {
+            (BoolOp::And, true) | (BoolOp::Or, false) => {
+                let mut aggregation: Option<NarrowingConstraints> = None;
+                for sub_constraint in sub_constraints.into_iter().flatten() {
+                    if let Some(ref mut some_aggregation) = aggregation {
+                        merge_constraints_and(some_aggregation, sub_constraint, self.db);
+                    } else {
+                        aggregation = Some(sub_constraint);
+                    }
+                }
+                aggregation
+            }
+            (BoolOp::Or, true) | (BoolOp::And, false) => {
+                let (first, rest) = sub_constraints.split_first_mut()?;
+                if let Some(ref mut first) = first {
+                    for rest_constraint in rest {
+                        if let Some(rest_constraint) = rest_constraint {
+                            merge_constraints_or(first, rest_constraint, self.db);
+                        } else {
+                            return None;
+                        }
+                    }
+                }
+                first.clone()
+            }
         }
     }
 }

crates/red_knot_python_semantic/src/types/unpacker.rs

@@ -0,0 +1,143 @@
+use std::borrow::Cow;
+
+use ruff_db::files::File;
+use ruff_python_ast::{self as ast, AnyNodeRef};
+use rustc_hash::FxHashMap;
+
+use crate::semantic_index::ast_ids::{HasScopedAstId, ScopedExpressionId};
+use crate::semantic_index::symbol::ScopeId;
+use crate::types::{TupleType, Type, TypeCheckDiagnostics, TypeCheckDiagnosticsBuilder};
+use crate::Db;
+
+/// Unpacks the value expression type to their respective targets.
+pub(crate) struct Unpacker<'db> {
+    db: &'db dyn Db,
+    targets: FxHashMap<ScopedExpressionId, Type<'db>>,
+    diagnostics: TypeCheckDiagnosticsBuilder<'db>,
+}
+
+impl<'db> Unpacker<'db> {
+    pub(crate) fn new(db: &'db dyn Db, file: File) -> Self {
+        Self {
+            db,
+            targets: FxHashMap::default(),
+            diagnostics: TypeCheckDiagnosticsBuilder::new(db, file),
+        }
+    }
+
+    pub(crate) fn unpack(&mut self, target: &ast::Expr, value_ty: Type<'db>, scope: ScopeId<'db>) {
+        match target {
+            ast::Expr::Name(target_name) => {
+                self.targets
+                    .insert(target_name.scoped_ast_id(self.db, scope), value_ty);
+            }
+            ast::Expr::Starred(ast::ExprStarred { value, .. }) => {
+                self.unpack(value, value_ty, scope);
+            }
+            ast::Expr::List(ast::ExprList { elts, .. })
+            | ast::Expr::Tuple(ast::ExprTuple { elts, .. }) => match value_ty {
+                Type::Tuple(tuple_ty) => {
+                    let starred_index = elts.iter().position(ast::Expr::is_starred_expr);
+
+                    let element_types = if let Some(starred_index) = starred_index {
+                        if tuple_ty.len(self.db) >= elts.len() - 1 {
+                            let mut element_types = Vec::with_capacity(elts.len());
+                            element_types.extend_from_slice(
+                                // SAFETY: Safe because of the length check above.
+                                &tuple_ty.elements(self.db)[..starred_index],
+                            );
+
+                            // E.g., in `(a, *b, c, d) = ...`, the index of starred element `b`
+                            // is 1 and the remaining elements after that are 2.
+                            let remaining = elts.len() - (starred_index + 1);
+                            // This index represents the type of the last element that belongs
+                            // to the starred expression, in an exclusive manner.
+                            let starred_end_index = tuple_ty.len(self.db) - remaining;
+                            // SAFETY: Safe because of the length check above.
+                            let _starred_element_types =
+                                &tuple_ty.elements(self.db)[starred_index..starred_end_index];
+                            // TODO: Combine the types into a list type. If the
+                            // starred_element_types is empty, then it should be `List[Any]`.
+                            // combine_types(starred_element_types);
+                            element_types.push(Type::Todo);
+
+                            element_types.extend_from_slice(
+                                // SAFETY: Safe because of the length check above.
+                                &tuple_ty.elements(self.db)[starred_end_index..],
+                            );
+                            Cow::Owned(element_types)
+                        } else {
+                            let mut element_types = tuple_ty.elements(self.db).to_vec();
+                            // Subtract 1 to insert the starred expression type at the correct
+                            // index.
+                            element_types.resize(elts.len() - 1, Type::Unknown);
+                            // TODO: This should be `list[Unknown]`
+                            element_types.insert(starred_index, Type::Todo);
+                            Cow::Owned(element_types)
+                        }
+                    } else {
+                        Cow::Borrowed(tuple_ty.elements(self.db).as_ref())
+                    };
+
+                    for (index, element) in elts.iter().enumerate() {
+                        self.unpack(
+                            element,
+                            element_types.get(index).copied().unwrap_or(Type::Unknown),
+                            scope,
+                        );
+                    }
+                }
+                Type::StringLiteral(string_literal_ty) => {
+                    // Deconstruct the string literal to delegate the inference back to the
+                    // tuple type for correct handling of starred expressions. We could go
+                    // further and deconstruct to an array of `StringLiteral` with each
+                    // individual character, instead of just an array of `LiteralString`, but
+                    // there would be a cost and it's not clear that it's worth it.
+                    let value_ty = Type::Tuple(TupleType::new(
+                        self.db,
+                        vec![Type::LiteralString; string_literal_ty.len(self.db)]
+                            .into_boxed_slice(),
+                    ));
+                    self.unpack(target, value_ty, scope);
+                }
+                _ => {
+                    let value_ty = if value_ty.is_literal_string() {
+                        Type::LiteralString
+                    } else {
+                        value_ty
+                            .iterate(self.db)
+                            .unwrap_with_diagnostic(AnyNodeRef::from(target), &mut self.diagnostics)
+                    };
+                    for element in elts {
+                        self.unpack(element, value_ty, scope);
+                    }
+                }
+            },
+            _ => {}
+        }
+    }
+
+    pub(crate) fn finish(mut self) -> UnpackResult<'db> {
+        self.targets.shrink_to_fit();
+        UnpackResult {
+            diagnostics: self.diagnostics.finish(),
+            targets: self.targets,
+        }
+    }
+}
+
+#[derive(Debug, Default, PartialEq, Eq)]
+pub(crate) struct UnpackResult<'db> {
+    targets: FxHashMap<ScopedExpressionId, Type<'db>>,
+    diagnostics: TypeCheckDiagnostics,
+}
+
+impl<'db> UnpackResult<'db> {
+    pub(crate) fn get(&self, expr_id: ScopedExpressionId) -> Option<Type<'db>> {
+        self.targets.get(&expr_id).copied()
+    }
+
+    pub(crate) fn diagnostics(&self) -> &TypeCheckDiagnostics {
+        &self.diagnostics
+    }
+}