from typing import Generic, TypeVar, Union

T = TypeVar('T')
class G(Generic[T]): pass
class A(object): pass
class B(object): pass
g_a = None  # type: G[A]
g_b = None  # type: G[B]

AB = TypeVar('AB', A, B)
def f(x):
    # type: (Union[G[AB],AB]) -> G[AB]
    pass

f(A())
f(B())
f(g_a)
f(g_b)  # E: Argument 1 to "f" has incompatible type G[B]; expected "Union[G[A], A]"