from typing import Optional, Union

import google.api_core.future.polling # type: ignore

#class A(google.api_core.future.polling.PollingFuture):
class A:
    pass

class B(A):
    pass

class C(A):
    pass

def subfunc(arg: Optional[str]) -> Union[B, C]:
    if arg is None:
        return B()
    else:
        return C()

def func_invalid() -> B:
    r = B()
    r = subfunc("arg")
    return r

def func_correct() -> B:
    r = B()
    r2 = subfunc("arg")
    assert isinstance(r2, B)
    return r2

func_invalid()
func_correct()