Wrapped methods are not properly resolved until after the class definition

Context & setup

Consider the following simple protocol:

class Proto(Protocol):
    @property
    def f(self) -> int:
        ...

One can implement this protocol wrongly as follows:

T = TypeVar("T")
P = ParamSpec("P")


def wrapper(f: Callable[P, T]) -> Callable[P, str]:
    def wrapped(*args: P.args, **kwargs: P.kwargs) -> str:
        return 0

    return wrapped

class Impl:
    @wrapper  # converts return type from `bool` to `str`, while the protocol expects `int`
    def f(self) -> bool:
        return False

# error: Incompatible return value type (got "Impl", expected "Proto")  [return-value]
# note: Following member(s) of "Impl" have conflicts:
# note:     Expected:
# note:         def f(self) -> int
# note:     Got:
# note:         def f(self) -> str
def b() -> Proto:
    return Impl()

The bug (false negative)

When this same function b is placed before class Impl, mypy does not report an error. Adding reveal_type(Impl.f) before and after the class definition hints at what is going on:

main.py:21: error: Expression has type "Any"  [misc]
main.py:21: error: Cannot determine type of "f"  [has-type]
main.py:21: note: Revealed type is "Any"
main.py:21: error: Name "Impl" is used before definition  [used-before-def]

main.py:28: note: Revealed type is "def (self: __main__.Impl) -> builtins.str"

If f is not decorated but explicitly returns str, one gets

main.py:21: note: Revealed type is "def (self: __main__.Impl) -> builtins.str"
main.py:21: error: Name "Impl" is used before definition  [used-before-def]

main.py:27: note: Revealed type is "def (self: __main__.Impl) -> builtins.str"

as expected. This indicates that the wrapped decorator is only properly resolved for code after the class definition.

False positive variant

If f is decorated with @property, this can also produce false positives. This is how I encountered the issue in the first place. Playground.

Additional info

Python: 3.11
Mypy: 371219347a6d17e16924bbabf3e693c6874e7138
Flags: --strict --disallow-any-*

リンクされた mypy-play の例を使って main.py の偽陰性と偽陽性を再現し、関数の配置、@wrapper、@property を比較する。class Impl の前後で decorator とラップされたメソッドの型がどのように解決されるかを追跡する。両方のバリエーションで、期待される protocol 互換性の診断が一貫して報告されれば完了とする。