Allow subclassing Callable

[ilevkivskyi]

This is not a priority, but probably we could allow subclassing Callable, some people might find this "nicer", than defining a __call__ method (e.g. if this is an ABC or a protocol). For example this already works at runtime:

class C(Callable[[int], int]):
    attr: str

This would be implemented by returning CallableType for C with a fallback to an instance of the original TypeInfo (like for tulpes, see also #3831).

[msullivan]

The protocol thing works fine

[bfontaine]

Why was this closed? Protocols are not sufficient because they don’t inherit callable properties like __name__ or __doc__:

from typing import Protocol

class MyFunction(Protocol):
    def __call__(self, x: int) -> int: ...

def print_name(fn: MyFunction):
    print(fn.__name__)

This fails with mypy 1.8.0:

test.py:9: error: "MyFunction" has no attribute "__name__"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)