Mypy should allow for Self in parametrized parent class definition — issues with contextlib.AbstractContextManager

Bug Report

AbstractContextManager defines a __enter__ that returns self

• https://github.com/python/typeshed/blob/6a8d653a671925b0a3af61729ff8cf3f90c9c662/stdlib/contextlib.pyi#L42
• https://github.com/python/cpython/blob/5436d8b9c397c48d9b0d5f9d4ad5e1d5a5d500f6/Lib/contextlib.py#L25

But it's impossible to just inherit from contextlib.AbstractContextManager and have the type of __enter__ be correctly assumed to be Self, without a bit of boilerplate.

To Reproduce

class A(contextlib.AbstractContextManager):
    pass

reveal_type(A().__enter__())  # should be Self but assumed to be Any

and mypy disallows writing

class B(contextlib.AbstractContextManager[Self]):  # error: Self type is only allowed in annotations within class definition  [misc]
    pass

and doesn't issue a warning for this runtime error:

class C(contextlib.AbstractContextManager[C]):
    pass

Full example here
https://mypy-play.net/?mypy=latest&python=3.11&gist=7e76e56be6c702005a7ffbb7846b69c2

Expected Behavior

Either Self is allowed in the AbstractContextManager parameter list, or the type vars work by default.

Actual Behavior

Errors. See the playground.
(Although pyright doesn't error when using Self in the parameter list, but it still wrongly infers Any as the return of __enter__)

Workaround

Repeat the class name, and repeat the __enter__ impelmentation

class E(contextlib.AbstractContextManager["E"]):
    def __enter__(self) -> Self:
        return self

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: n/a
• Python version used: 3.11

Inizia riproducendo gli esempi nel link mypy-play, in particolare i casi AbstractContextManager e Self, e confronta i tipi rivelati e gli errori segnalati con il comportamento previsto. Il lavoro è completato quando un test di regressione mirato dimostra il comportamento scelto di Self o della variabile di tipo senza richiedere il workaround, preservando al contempo la semantica rilevante di Python 3.11.