Cryptic error message for two generic callables with different variable binding

Bug Report

If a @contextmanager yields a generic type alias, mypy 1.11 errors with Argument 1 to "contextmanager" has incompatible type "$TYPE"; expected "$TYPE" [arg-type], where the inferred and expected types are identical.

This does not happen if the generic type is inlined (no type alias), or if the type alias is not generic; see playground link below for examples.

This started happening in 1.11 (didn't happen in 1.10). This may be related to https://github.com/python/mypy/issues/8696, but I opened a new issue because it looks like a regression.

Real code that tripped this: link.

To Reproduce

Short version (more examples in playground links below):

GenericFn = Callable[[_T], _T]

@contextmanager
def make_generic() -> Iterator[GenericFn[_T]]:
    def rv(obj: _T) -> _T:
        return obj
    yield rv

Playground:

• 1.11.0 fails
• 1.10.1 succeeds

Expected Behavior

Either fail with a useful error message (the inferred and expected types being exactly the same is confusing; I am not sure what would fix the error), or don't fail (like 1.10).

Actual Behavior

error: Argument 1 to "contextmanager" has incompatible type "Callable[[], Iterator[Callable[[_T], _T]]]"; expected "Callable[[], Iterator[Callable[[_T], _T]]]"  [arg-type]

Your Environment

• Mypy version used: 1.11.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.12

Reproduisez l’exemple minimal de @contextmanager avec mypy 1.11.0 sous Python 3.12 et --strict, puis comparez-le avec 1.10.1 et les cas liés du playground. Suivez la gestion des alias de types génériques autour de make_generic et contextmanager ; le travail est terminé lorsque la régression est évitée ou que les types inféré et attendu identiques sont remplacés par une erreur utile.