Calling a method of Union of two classes with class's attribute results in incompatible type error

[Jackenmen]

Honestly didn't know how to name this issue...

Issue type: Bug
Problematic code:

from typing import Union

class UserTypeA: ...
class UserTypeB: ...

class A:
    def permissions_for(self, user: UserTypeA) -> bool: ...
    
    @property
    def me(self) -> UserTypeA: ...

class B:
    def permissions_for(self, user: UserTypeB) -> bool: ...
    
    @property
    def me(self) -> UserTypeB: ...


def func(x: Union[A, B]) -> bool:
    return x.permissions_for(x.me)

Error message:

main.py:20: error: Argument 1 to "permissions_for" of "A" has incompatible type "Union[UserTypeA, UserTypeB]"; expected "UserTypeA"
main.py:20: error: Argument 1 to "permissions_for" of "B" has incompatible type "Union[UserTypeA, UserTypeB]"; expected "UserTypeB"

Expected behaviour:
No error as x.me matches the type expected by x.permissions_for - if instance of A is passed x.me will have type UserTypeA and x.permissions_for will expect UserTypeA and same for instance of B.

Also, if func looks like this, the errors are gone so if mypy could infer this without isinstance it would be great:

def func(x: Union[A, B]) -> bool:
    if isinstance(x, A):
        return x.permissions_for(x.me)
    else:
        return x.permissions_for(x.me)

Python version:: 3.8.1
Mypy version: 0.761, occurs on master as well
Mypy flags: No flags

[JukkaL]

This is probably too difficult for mypy to understand, but leaving open as a low-priority issue in case somebody comes up with a simple way to support this.

[intgr]

Meanwhile you could solve this by creating a Protocol for the common methods shared between A and B, and taking that as argument.