Impossible to specify a metaclass for a Generic class

[rowillia]

Not sure if this issue should go here or in https://github.com/python/typing

Below is a contrived example. In the real world I might not actually control the metaclass for Foo or I may be inheriting from some other class that also has a metaclass.

from typing import Generic, TypeVar

class FooMeta(type):
    pass

_T = TypeVar('_T')

class Foo(Generic[_T], metaclass=FooMeta):
    pass

This passes MyPy just fine, but fails at runtime with:

$ python3.6 test_generics.py
Traceback (most recent call last):
  File "test_generics.py", line 8, in <module>
    class Foo(Generic[_T], metaclass=FooMeta):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

[ilevkivskyi]

I think this should be in typing rather than here. (Also this is unfortunately not an easy problem.)

[rowillia]

Thanks @ilevkivskyi , moving over there.