Allow `types.GeneratorType` to be used instead of `collections.abc.Generator` as the return type of generator functions

Feature

Currently, mypy only allows collections.abc.Generator and its supertypes as the return type of generator functions, and as a result, it cannot infer the correct return type for awaitable objects that define the __await__() method via types.GeneratorType:

import asyncio

from collections.abc import Generator
from types import GeneratorType
from typing import Never, reveal_type


class A:
    def __await__(self) -> GeneratorType[Never, Never, bool]:
        return True
        yield  # generator definition


class B:
    def __await__(self) -> Generator[Never, Never, bool]:
        return True
        yield  # generator definition


async def main() -> None:
    reveal_type(await A())  # revealed type is "Any"
    reveal_type(await B())  # revealed type is "builtins.bool"

    reveal_type(A().__await__().gi_running)  # revealed type is "builtins.bool"
    reveal_type(B().__await__().gi_running)  # has no attribute "gi_running"


if __name__ == "__main__":
    asyncio.run(main())

example.py:9: error: The return type of a generator function should be "Generator" or one of its supertypes  [misc]
example.py:21: note: Revealed type is "Any"
example.py:22: note: Revealed type is "builtins.bool"
example.py:24: note: Revealed type is "builtins.bool"
example.py:25: error: "Generator[Never, Never, bool]" has no attribute "gi_running"  [attr-defined]
example.py:25: note: Revealed type is "Any"
Found 2 errors in 1 file (checked 1 source file)

I propose allowing the use of types.GeneratorType with subsequent inference of the correct return type.

Pitch

First, pyright already has the expected behavior:

/home/user/workspace/example.py
  /home/user/workspace/example.py:21:17 - information: Type of "await A()" is "bool"
  /home/user/workspace/example.py:22:17 - information: Type of "await B()" is "bool"
  /home/user/workspace/example.py:24:17 - information: Type of "A().__await__().gi_running" is "bool"
  /home/user/workspace/example.py:25:17 - information: Type of "B().__await__().gi_running" is "Unknown"
  /home/user/workspace/example.py:25:33 - error: Cannot access attribute "gi_running" for class "Generator[Never, Never, bool]"
    Attribute "gi_running" is unknown (reportAttributeAccessIssue)
1 error, 0 warnings, 4 informations

Second, this relates to the eternally controversial python/typing#1480. In python/typeshed#10816, false properties related to native objects were removed in favor of the types module. You could say that we can use isinstance/inspect.isgenerator to narrow collections.abc.Generator down to types.GeneratorType, but... it is not that simple:

    x = B().__await__()
    reveal_type(x)  # typing.Generator[Never, Never, builtins.bool]

    if isgenerator(x):
        reveal_type(x)  # types.GeneratorType[Any, Any, Any]

    if isinstance(x, GeneratorType):
        reveal_type(x)  # types.GeneratorType[Any, Any, Any]

I think that allowing the use of types.GeneratorType would make it easier to inspect native generator objects. Especially since collections.abc.Generator can also be used to define custom generator types.

Related: #18635.

Beginne mit dem example.py-Fall im Issue und vergleiche die aktuellen Diagnosen von mypy mit dem erwarteten Verhalten von pyright. Die Arbeit ist abgeschlossen, wenn types.GeneratorType als Rückgabetyp einer Generatorfunktion akzeptiert wird und die gezeigten Typen von await und Generatorattributen wie erwartet inferiert werden.