Lambda properties on objects don't typecheck correctly (mypy assumes self is passed)

[ezyang]

I wanted to store a lambda in an object and then call it. Mypy doesn't like it.

from typing import Callable

class A:
    f: Callable[[bool], bool]
    def __init__(self, f):
        self.f = f

def foo(b: bool) -> bool:
    return b

a = A(foo)
print(a.f(True))

Running:

dhcp-172-31-207-247:~ ezyang$ python3.7 bb.py 
True
dhcp-172-31-207-247:~ ezyang$ mypy --version
mypy 0.660
dhcp-172-31-207-247:~ ezyang$ mypy bb.py
bb.py:12: error: Invalid self argument "A" to attribute function "f" with type "Callable[[bool], bool]"
bb.py:12: error: Too many arguments

(The reason I want to do this is because the object is a dataclass and I'm not exactly sure how well inheritance works with dataclasses, so I thought, maybe I can keep things simple by just storing callables on the object.)

[ilevkivskyi]

This is actually a duplicate of #708 (although this may be non-obvious), see also #6910