Unsound assignment of a method to a Callable

Bug Report

Assigning functions to a name that was defined as a Callable is unsound and results in TypeErrors when run in Python.

To Reproduce

Just typecheck the code below with --strict:

class B():
    a: Callable[['B', int], int]
    def b(self, i: int) -> int: return 1
    a = b

B().a(B(), 1)

results in no errors.

Expected Behavior

Python however raises a TypeError for the same code:

>>> B().a(B(), 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: b() takes 2 positional arguments but 3 were given

I feel like Mypy should probably somdisallow assigning functions in the class body to Callable

Your Environment

• Mypy version used: 9e1f4df133e155f213cf3714bf796bb9e8698907 (> v1.5.1)
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10

No source file or test is named. First run the supplied example with mypy --strict and compare it with Python's TypeError; then trace how class-body assignments to Callable annotations are checked. Done means the unsound assignment is rejected and a regression test covers the example.