I believe I'm experiencing a related bug in the new (3.7) version unfortunately. The current version of typing.Generic.__init_subclass__ isn't chaining to super(Generic, cls).__init_subclass__, meaning Generic's position in class bases can prevent subsequent bases from working properly. I can currently work around it with this unsightly hack but it's obviously suboptimal:

    _old_generic_init_subclass = object.__getattribute__(ta.Generic, '__init_subclass__').__func__
    @classmethod  # noqa
    def _new_generic_init_subclass(cls, *args, **kwargs):  # noqa
        _old_generic_init_subclass(cls, *args, **kwargs)
        super(ta.Generic, cls).__init_subclass__(*args, **kwargs)
    ta.Generic.__init_subclass__ = _new_generic_init_subclass  # noqa