Function signature matching fails at import

[jolaf]

Bug Report

The provided test defines a function with the same type signature in two ways, directly and through import.

mypy objects that functions are incompatible.

To Reproduce

https://gist.github.com/mypy-play/a5f51cd9246e6478516af53493402890

[brianschubert]

This isn't clear from the error message, but the reason for the error is that the signatures actually are different. The two definitions have different (positional-or-keyword) parameter names:

reveal_type(x)              # note: Revealed type is "def [_F <: def (*Any, **Any) -> Any] (f: _F`-1) -> _F`-1"
reveal_type(no_type_check)  # note: Revealed type is "def [_F <: def (*Any, **Any) -> Any] (arg: _F`-1) -> _F`-1"

If you change the local definition of x to use the same parameter name as no_type_check, the code will type check:

if b:
    def x(arg: _F) -> _F:
        return arg
else:
    from typing import no_type_check as x  # ok

We can probably improve the error message to render the pretty callable signatures, similar to what we do for conditional override mismatches without the import:

if b:
    def x(f: _F) -> _F:
        return f
else:
    def x(arg: _F) -> _F: 
        return arg

error: All conditional function variants must have identical signatures  [misc]
note: Original:
note:     def [_F: Callable[..., Any]] x(f: _F) -> _F
note: Redefinition:
note:     def [_F: Callable[..., Any]] x(arg: _F) -> _F

[jolaf]

Well, at least diagnostics should be fixed, because without that this situation really looks like a bug in mypy.

Also I wonder is there a way to tell mypy that I don't care about argument names because the function is a decorator and the argument is always passed positionally.

[brianschubert]

Also I wonder is there a way to tell mypy that I don't care about argument names because the function is a decorator and the argument is always passed positionally.

If you make the parameter positional-only, the name won't be considered: https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=9e7d630d2cd5a0e52d7de50da714f6fb

[jolaf]

Wow, I didn't know this syntax, thank you!