Base class function signature compatibility checks overly strict

[ambv]

Actual example of this code can be found in Python 3.6's enum with Flag and IntFlag.

Minimal repro::

from typing import TypeVar

_T = TypeVar('_T')


class Base:
    def __or__(self: _T, other: _T) -> _T: ...
    def __and__(self: _T, other: _T) -> _T: ...
    def __xor__(self: _T, other: _T) -> _T: ...


class Other(int, Base):  # <-- errors here
    def __or__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore
    def __and__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore
    def __xor__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore

Expected behavior: no errors.
Actual behavior:

f:11: error: Definition of "__and__" in base class "int" is incompatible with definition in base class "Base"
f:11: error: Definition of "__or__" in base class "int" is incompatible with definition in base class "Base"
f:11: error: Definition of "__xor__" in base class "int" is incompatible with definition in base class "Base"

The errors would be fine if Other didn't redefine the methods itself. In this case the incompatibility in base classes is irrelevant.

[JukkaL]

Keywords to help searching: self type, union type

[jingw]

Just ran into this at Dropbox.

Slightly more distilled repro:

class A(object):
    def x(self):
        # type: () -> A
        return self

class B(object):
    def x(self):
        # type: () -> B
        return self

class C(A, B):
    def x(self):
        # type: () -> C
        return self

test.py:11: error: Definition of "x" in base class "A" is incompatible with definition in base class "B"

And equivalent compilable Java code:

interface A {
    A x();
}
interface B {
    B x();
}
class C implements A, B {
    public C x() {
        return this;
    }
}

I presume mypy already has checks that C.x is compatible with A.x and B.x. Is it possible to resolve this issue by doing something along the lines of removing the check that A.x and B.x are compatible in the case where x is overridden? (Note, I don't really know what I'm talking about.)