Functions can't be passed as callable protocol

[srittau]

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.

[ilevkivskyi]

This is a great idea! And it is easy to implement. TBH I never liked NamedArg, KwArg, etc. This will look much cleaner and flexible (because we can even express overloads as callbacks).

[srittau]

Credit for this idea goes to @podhmo. Interestingly, typescript does something similar. While it is possible to define callables just using

type MyCallback = (foo: number) => string

or similar, you can define overloaded functions like this:

interface MyCallback {
    () => number;
    (foo: number) => string;
}