(🐞) `Protocol` compatibility is broken for `__call__(self, *args: Any)`

# mypy: disable-error-code=empty-body
from __future__ import annotations

from typing import Any, Protocol

class A(Protocol):
    def __call__(self, *args: Any) -> None: ...

class B1(A):
    def __call__(self, x: int) -> None: ...  # no error

class B2:
    def __call__(self, x: int) -> None: ...

a: A = B2()  # Incompatible types in assignment (expression has type "B2", variable has type "A")  [assignment]

@ilevkivskyi

Start by running the provided Python reproducer through mypy and compare the Protocol-based A/B1 case with the structural B2 assignment. Trace the callable Protocol compatibility check that produces the assignment diagnostic. Done means B2 is accepted as an A while the existing incompatible cases remain correctly rejected.