Cryptic error message for two generic callables with different variable binding

[lemon24]

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 #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

[JukkaL]

The behavior changed in #17348.

@ilevkivskyi Do you have an idea of what is going on?

[ilevkivskyi]

This now "correctly" exposes an old bug #3924. When I print the actual and expected types, they are def [T] () -> Iterator[def (T) -> T] and def () -> Iterator[def [T] (T) -> T] so the error is technically correct, as the former is indeed not a subtype of the latter. The problem is that with current type printing logic, both callables have same user-facing representation.

Actually, I think we should finally fix #3924, although it has only few upvotes, it affects much more people, there are a lot of duplicates since it is hard to track these kind of errors to the original issue. I am also leaving this one open for bad error message.