This came up in python/typeshed#2379. Consider the following code:

from typing import Protocol

class Caller(Protocol):
    def __call__(self) -> None: ...

def call() -> None:
    pass

def func(caller: Caller) -> None:
    pass

func(call)

Currently, mypy 0.620 rejects this:

foo.py:12: error: Argument 1 to "func" has incompatible type "Callable[[], None]"; expected "Caller"

I believe that mypy should actually allow this, because call is actually an object that fulfills the protocol. This would also open up the opportunity to create callback signatures of any complexity (for example, default and keyword-only arguments) without the need for additional features for Callable.