Positional-or-kw arg considered incompatible with positional-only argument #12525

Bug Report

from typing_extensions import Protocol


class Func(Protocol):
    def __call__(self, __a: int, **kwargs: str) -> None:
        #              ^^^ positional-only arg
        ...
    # Same error when using PEP 570 positional-only syntax:
    # def __call__(self, a: int, /, **kwargs: str) -> None:


def myfunc(a: int, **kwargs: str) -> None:
    pass


f: Func = myfunc

Expected Behavior

In my view this should be allowed. Anyone calling f() promises to pass the 1st argument as positional. And myfunc accepts the 1st argument as positional or kwarg, so it should be considered compatible.

Actual Behavior

Mypy output:

positional_only.py:12: error: Incompatible types in assignment (expression has type "Callable[[int, KwArg(str)], Any]", variable has type "Func")
positional_only.py:12: note: "Func.__call__" has type "Callable[[int, KwArg(str)], Any]"

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.10.2
• Operating system and version: macOS