Incomplete inference of extracted values from union of tuples

I expect this is a feature request, though it's possible it indicates a bug.

from typing import Union, Tuple

def func() -> Union[Tuple[None, None], Tuple[int, int]]:
    ...

a, b = func()

if a:
    print(a + b)

if a is not None:
    print(a + b)

# Unsupported operand types for + ("int" and "None")
# Right operand is of type "Optional[int]"

Both if tests result in the same behaviour.

Ideally I'd like mypy to know that the test here for a is sufficient to also know that b cannot be None (due to the way that Union works).

Not sure if this is asking too much though.

Python 3.6, mypy 0.770, also master (at 2a3de7b7941f24b6bc53fe0cf8a2b7f8f7fd4e15)

Reproduce el ejemplo de Python con mypy y compara los diagnósticos de las dos comprobaciones sobre un valor inferido a partir de la unión de tuplas. Investiga los puntos de entrada del estrechamiento de tipos implicados en la inferencia de uniones de tuplas. Se considera terminado cuando ambas comprobaciones establecen que b es un int y dejan de informar de un tipo de operando no compatible.