From e170fe493dfdffbb833629d7565a5e0555c528a7 Mon Sep 17 00:00:00 2001
From: David Peter <sharkdp@users.noreply.github.com>
Date: Wed, 23 Apr 2025 20:07:29 +0200
Subject: [PATCH] [red-knot] Trust all symbols in stub files (#17588)

## Summary

*Generally* trust undeclared symbols in stubs, not just at the module
level.

Follow-up on the discussion
[here](https://github.com/astral-sh/ruff/pull/17577#discussion_r2055945909).

## Test Plan

New Markdown test.
---
 .../mdtest/boundness_declaredness/public.md   | 63 +++++++++++++++++++
 .../src/semantic_index/symbol.rs              |  8 ---
 crates/red_knot_python_semantic/src/symbol.rs |  2 +-
 3 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md b/crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md
index 3c41c72e44..956fa1cc7f 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md
@@ -292,3 +292,66 @@ reveal_type(a)  # revealed: Unknown
 # Modifications allowed in this case:
 a = None
 ```
+
+## In stub files
+
+In stub files, we have a minor modification to the rules above: we do not union with `Unknown` for
+undeclared symbols.
+
+### Undeclared and bound
+
+`mod.pyi`:
+
+```pyi
+MyInt = int
+
+class C:
+    MyStr = str
+```
+
+```py
+from mod import MyInt, C
+
+reveal_type(MyInt)  # revealed: Literal[int]
+reveal_type(C.MyStr)  # revealed: Literal[str]
+```
+
+### Undeclared and possibly unbound
+
+`mod.pyi`:
+
+```pyi
+def flag() -> bool:
+    return True
+
+if flag():
+    MyInt = int
+
+    class C:
+        MyStr = str
+```
+
+```py
+# error: [possibly-unbound-import]
+# error: [possibly-unbound-import]
+from mod import MyInt, C
+
+reveal_type(MyInt)  # revealed: Literal[int]
+reveal_type(C.MyStr)  # revealed: Literal[str]
+```
+
+### Undeclared and unbound
+
+`mod.pyi`:
+
+```pyi
+if False:
+    MyInt = int
+```
+
+```py
+# error: [unresolved-import]
+from mod import MyInt
+
+reveal_type(MyInt)  # revealed: Unknown
+```
diff --git a/crates/red_knot_python_semantic/src/semantic_index/symbol.rs b/crates/red_knot_python_semantic/src/semantic_index/symbol.rs
index 10f6e9c5d1..ec9ef3886e 100644
--- a/crates/red_knot_python_semantic/src/semantic_index/symbol.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/symbol.rs
@@ -115,10 +115,6 @@ impl<'db> ScopeId<'db> {
         self.node(db).scope_kind().is_function_like()
     }
 
-    pub(crate) fn is_module_scope(self, db: &'db dyn Db) -> bool {
-        self.node(db).scope_kind().is_module()
-    }
-
     pub(crate) fn is_type_parameter(self, db: &'db dyn Db) -> bool {
         self.node(db).scope_kind().is_type_parameter()
     }
@@ -267,10 +263,6 @@ impl ScopeKind {
         matches!(self, ScopeKind::Class)
     }
 
-    pub(crate) fn is_module(self) -> bool {
-        matches!(self, ScopeKind::Module)
-    }
-
     pub(crate) fn is_type_parameter(self) -> bool {
         matches!(self, ScopeKind::Annotation | ScopeKind::TypeAlias)
     }
diff --git a/crates/red_knot_python_semantic/src/symbol.rs b/crates/red_knot_python_semantic/src/symbol.rs
index 4e713a7fde..3be4ce8d02 100644
--- a/crates/red_knot_python_semantic/src/symbol.rs
+++ b/crates/red_knot_python_semantic/src/symbol.rs
@@ -593,7 +593,7 @@ fn symbol_by_id<'db>(
                 "__slots__" | "TYPE_CHECKING"
             );
 
-            if scope.is_module_scope(db) && scope.file(db).is_stub(db.upcast()) {
+            if scope.file(db).is_stub(db.upcast()) {
                 // We generally trust module-level undeclared symbols in stubs and do not union
                 // with `Unknown`. If we don't do this, simple aliases like `IOError = OSError` in
                 // stubs would result in `IOError` being a union of `OSError` and `Unknown`, which