Decorator becomes untyped when it accepts an object of the same type as a certain global.

Bug Report

A decorator becomes untyped when it accepts an argument for which we have another instance, globally defined and referred to in the same scope.

The comments in the example should make the bug much more clear.
I had no idea how to describe this issue or search for it, so no idea whether it was reported before.

To Reproduce

from typing import Callable


class Something:
    pass


def decorator(buffer: Something) -> Callable[[Callable[[], None]], Callable[[], None]]:
    return lambda x: x


def main() -> None:
    something  # Comment out this line and everything is fine.
    another_something = Something()

    @decorator(another_something)
    def func() -> None:
        return


something = Something()  # Move this above `main` and everything is fine.

Expected Behavior

The code snippet should pass.

Actual Behavior

 $ mypy --strict /tmp/test.py
test.py:16: error: Untyped decorator makes function "func" untyped
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.982
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.13

まず、mypy --strict を使ってスニペットを再現し、グローバルな something を削除した場合、または main より上に移動した場合の挙動を比較します。この例に関係するデコレーターの型推論と名前解決の経路を追跡します。元のスニペットが、型付けされていないデコレーターを報告せずに strict チェックを通過すれば完了です。