Argument incompatibility not detected with multiple generic base classes

• Are you reporting a bug, or opening a feature request?

Reporting a bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

Minimal repro:

from abc import ABCMeta, abstractmethod
from typing import Generic, TypeVar, Union

A = TypeVar('A')


class Base(Generic[A]):
    __metaclass__ = ABCMeta

    @abstractmethod
    def do_stuff(self, a):
        # type: (A) -> None
        pass


class Foo(Generic[A], Base[A]):
    pass


class Bar(Generic[A], Base[A]):
    pass


class Baz(Foo[str], Bar[Union[str, int]]):
    def do_stuff(self, a):
        # type: (str) -> None
        return None

• What is the actual behavior/output?

No error is reported

• What is the behavior/output you expect?

The definition of Baz.do_stuff() accepts a str, which is incompatible with Bar.do_stuff()'s abstract method definition (which accepts Union[str, int]).

This bug appears to be related to the following TODO comment: https://github.com/python/mypy/blob/d1c8f27f696012085ca904e96f8b93b92b74919e/mypy/checker.py#L1634

Interestingly, if I reverse the order of Foo and Bar in Baz's base class list, mypy reports the error.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

https://github.com/python/mypy/tree/103f5f3ec3fcc8fba429edcc4ee61c1370153ff9

Start with the TODO in mypy/checker.py around line 1634, then reproduce the issue using the minimal Foo, Bar, and Baz example in this report. The fix is complete when mypy reports the incompatible argument for Baz regardless of whether Foo or Bar appears first in the base class list.