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 the Python example with mypy and compare the diagnostics for the two checks on a value inferred from the union of tuples. Investigate the type-narrowing entry points involved in tuple-union inference. Done means both checks establish that b is an int and no longer report an unsupported operand type.