I've noticed this problem, very similar to #3262 , except with overloaded operators.

Originally, #3262 made it so that code like this:

from typing import overload

class A:
    def f(self, x : 'A') -> 'A': ...

class B(A):
    @overload
    def f(self, x : 'A') -> 'A': ...
    @overload
    def f(self, x : 'B') -> 'B' : ...

Wouldn't typecheck.

#3263 appears to have fixed this. Unfortunately, while this works for normal method signatures, it doesn't appear to work for overloaded operators. So the very similar code:

from typing import overload

class A:
    def __add__(self, x : 'A') -> 'A': ...

class B(A):
    @overload
    def __add__(self, x : 'A') -> 'A': ...
    @overload
    def __add__(self, x : 'B') -> 'B' : ...

Still fails to typecheck, with the error message:

test.pyi:7: error: Signature of "__add__" incompatible with supertype "A"

I can't think of any reason why this should be a problem with operators, although maybe someone else can explain.

I'm on Python 3.6.3 and MyPy 0.600+dev (master branch from git)