Regression in 1.2.0: callback protocol with Callable and Concatenate

[Rogdham]

Bug Report

The code below passed with mypy 1.1.1 but not in 1.2.0

To Reproduce

from typing import Concatenate, Callable, ParamSpec, Protocol

P = ParamSpec("P")


class Decorator(Protocol):
    def __call__(self, fct: Callable[Concatenate[str, P], int]) -> Callable[P, int]:
        ...


def foo(value: int) -> Decorator:
    def bar(fct: Callable[Concatenate[str, P], int]) -> Callable[P, int]:
        raise NotImplementedError

    return bar

I haven't been able to strip down any further, it seems that it has something to do with both Callable and Concatenate.

Expected Behavior

No error (as in mypy 1.1.1).

Actual Behavior

The following error is displayed:

issue.py:15: error: Incompatible return value type (got "Callable[[Callable[[str, **P], int]], Callable[P, int]]", expected "Decorator")  [return-value]
issue.py:15: note: "Decorator.__call__" has type "Callable[[Arg(Callable[[str, **P], int], 'fct')], Callable[P, int]]"
Found 1 error in 1 file (checked 1 source file)

Note that I have tried to change the Decorator definition to make fct parameter positional-only:

class Decorator(Protocol):
    def __call__(self, fct: Callable[Concatenate[str, P], int], /) -> Callable[P, int]:
        ...

Which resulted in the error message being even more confusing:

issue.py:15: error: Incompatible return value type (got "Callable[[Callable[[str, **P], int]], Callable[P, int]]", expected "Decorator")  [return-value]
issue.py:15: note: "Decorator.__call__" has type "Callable[[Callable[[str, **P], int]], Callable[P, int]]"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.2.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.3

[Rogdham]

Possibly related to #15037 in which the same error arises without the callback protocol part, like so:

def foo(value: int) -> Callable[[Callable[Concatenate[str, P], int]], Callable[P, int]]:
    def bar(fct: Callable[Concatenate[str, P], int]) -> Callable[P, int]:
        raise NotImplementedError

    return bar

issue.py:10: error: Incompatible return value type (got "Callable[[Arg(Callable[[str, **P], int], 'fct')], Callable[P, int]]", expected "Callable[[Callable[[str, **P], int]], Callable[P, int]]")  [return-value]
Found 1 error in 1 file (checked 1 source file)

[AlexWaygood]

This bisects to 730ba8a (cc. @A5rocks)

[AlexWaygood]

This bisects to 730ba8a (cc. @A5rocks)

But #14903 (by @A5rocks) appears to fix the issue! (The PR has not yet been merged, and looks like it still has a few issues for now -- e.g. there's a crash in the latest mypy_primer diff.)

[Rogdham]

Thank you @AlexWaygood for your analysis!

In that case it seems to confirm that it is a duplicate of #15037. But at least it gives @A5rocks an other case (the example from the description of the PR uses a Protocol) to test their PR with!

[abid-mujtaba]

Looks like the fix was reverted: #15272 (comment). Can we please re-open this issue so that we can keep track of the problem? Thanks.

[A5rocks]

Fortunately, the PR it was reverted for fixes this too.

Namely, #15837