Specilizing Union method parameters on subclasses

[athoscouto]

Feature Request
Extend feature added in #424 to allow subclasses to also specialize method parameters that contain Union type.

Code example

class A():
    def x(self, x: Union[Dict, List]):
        pass


class B(A):
    def x(self, x: Dict):
        pass

mypy output
error: Argument 1 of "x" incompatible with supertype "A"

Expected output
Nothing :)

mypy/python version
mypy 0.641
Python 3.6.7

[intgr]

I think this issue is mistaken. The contract of A states "all A classes contain method x that accepts argument Union[Dict, List]".

The method B.x violates that contract because it does not accept List.

[intgr]

This is basically duplicate of #1237.

[JukkaL]

Yeah, the method in B violates Liskov substitution principle (https://en.wikipedia.org/wiki/Liskov_substitution_principle).