[ty] emit diagnostics for method definitions and other invalid statements in TypedDict class bodies (#22351)

Fixes https://github.com/astral-sh/ty/issues/2277.

crates/ty_python_semantic/resources/mdtest/override.md

@@ -551,10 +551,8 @@ class MyNamedTupleChild(MyNamedTupleParent):
 class MyTypedDict(TypedDict):
     x: int
 
+    # error: [invalid-typed-dict-statement] "TypedDict class cannot have methods"
     @override
-    # TODO: it's invalid to define a method on a `TypedDict` class,
-    # so we should emit a diagnostic here.
-    # It shouldn't be an `invalid-explicit-override` diagnostic, however.
     def copy(self) -> Self: ...
 
 class Grandparent(Any): ...

crates/ty_python_semantic/resources/mdtest/snapshots/typed_dict.md_-_`TypedDict`_-_Only_annotated_decla…_(bef70731cae5b8af).snap

@@ -0,0 +1,103 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+
+---
+mdtest name: typed_dict.md - `TypedDict` - Only annotated declarations are allowed in the class body
+mdtest path: crates/ty_python_semantic/resources/mdtest/typed_dict.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing import TypedDict
+ 2 | 
+ 3 | class Foo(TypedDict):
+ 4 |     """docstring"""
+ 5 | 
+ 6 |     annotated_item: int
+ 7 |     """attribute docstring"""
+ 8 | 
+ 9 |     pass
+10 | 
+11 |     # As a non-standard but common extension, we interpret `...` as equivalent to `pass`.
+12 |     ...
+13 | 
+14 | class Bar(TypedDict):
+15 |     a: int
+16 |     # error: [invalid-typed-dict-statement] "invalid statement in TypedDict class body"
+17 |     42
+18 |     # error: [invalid-typed-dict-statement] "TypedDict item cannot have a value"
+19 |     b: str = "hello"
+20 |     # error: [invalid-typed-dict-statement] "TypedDict class cannot have methods"
+21 |     def bar(self): ...
+22 | class Baz(Bar):
+23 |     # error: [invalid-typed-dict-statement]
+24 |     def baz(self):
+25 |         pass
+```
+
+# Diagnostics
+
+```
+error[invalid-typed-dict-statement]: invalid statement in TypedDict class body
+  --> src/mdtest_snippet.py:17:5
+   |
+15 |     a: int
+16 |     # error: [invalid-typed-dict-statement] "invalid statement in TypedDict class body"
+17 |     42
+   |     ^^
+18 |     # error: [invalid-typed-dict-statement] "TypedDict item cannot have a value"
+19 |     b: str = "hello"
+   |
+info: Only annotated declarations (`<name>: <type>`) are allowed.
+info: rule `invalid-typed-dict-statement` is enabled by default
+
+```
+
+```
+error[invalid-typed-dict-statement]: TypedDict item cannot have a value
+  --> src/mdtest_snippet.py:19:14
+   |
+17 |     42
+18 |     # error: [invalid-typed-dict-statement] "TypedDict item cannot have a value"
+19 |     b: str = "hello"
+   |              ^^^^^^^
+20 |     # error: [invalid-typed-dict-statement] "TypedDict class cannot have methods"
+21 |     def bar(self): ...
+   |
+info: rule `invalid-typed-dict-statement` is enabled by default
+
+```
+
+```
+error[invalid-typed-dict-statement]: TypedDict class cannot have methods
+  --> src/mdtest_snippet.py:21:5
+   |
+19 |     b: str = "hello"
+20 |     # error: [invalid-typed-dict-statement] "TypedDict class cannot have methods"
+21 |     def bar(self): ...
+   |     ^^^^^^^^^^^^^^^^^^
+22 | class Baz(Bar):
+23 |     # error: [invalid-typed-dict-statement]
+   |
+info: rule `invalid-typed-dict-statement` is enabled by default
+
+```
+
+```
+error[invalid-typed-dict-statement]: TypedDict class cannot have methods
+  --> src/mdtest_snippet.py:24:5
+   |
+22 |   class Baz(Bar):
+23 |       # error: [invalid-typed-dict-statement]
+24 | /     def baz(self):
+25 | |         pass
+   | |____________^
+   |
+info: rule `invalid-typed-dict-statement` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/typed_dict.md

@@ -2266,6 +2266,47 @@ def match_with_dict(u: Foo | Bar | dict):
             reveal_type(u)  # revealed: Foo | (dict[Unknown, Unknown] & ~<TypedDict with items 'tag'>)
 ```
 
+## Only annotated declarations are allowed in the class body
+
+<!-- snapshot-diagnostics -->
+
+`TypedDict` class bodies are very restricted in what kinds of statements they can contain. Besides
+annotated items, the only allowed statements are docstrings and `pass`. Annotated items are are also
+not allowed to have a value.
+
+```py
+from typing import TypedDict
+
+class Foo(TypedDict):
+    """docstring"""
+
+    annotated_item: int
+    """attribute docstring"""
+
+    pass
+
+    # As a non-standard but common extension, we interpret `...` as equivalent to `pass`.
+    ...
+
+class Bar(TypedDict):
+    a: int
+    # error: [invalid-typed-dict-statement] "invalid statement in TypedDict class body"
+    42
+    # error: [invalid-typed-dict-statement] "TypedDict item cannot have a value"
+    b: str = "hello"
+    # error: [invalid-typed-dict-statement] "TypedDict class cannot have methods"
+    def bar(self): ...
+```
+
+These rules are also enforced for `TypedDict` classes that don't directly inherit from `TypedDict`:
+
+```py
+class Baz(Bar):
+    # error: [invalid-typed-dict-statement]
+    def baz(self):
+        pass
+```
+
 [closed]: https://peps.python.org/pep-0728/#disallowing-extra-items-explicitly
 [subtyping section]: https://typing.python.org/en/latest/spec/typeddict.html#subtyping-between-typeddict-types
 [`typeddict`]: https://typing.python.org/en/latest/spec/typeddict.html