(inter-)dependent variable types

I'm not sure how to entitle this, but I think it's a dependent type.

• Are you reporting a bug, or opening a feature request? yes
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Tuple, Union

def x() -> Union[Tuple[int, int], Tuple[str, str]]:
    ...

a, b = x()

if isinstance(b, int):
    reveal_type(a) # can only be int but is reported as Union[int, str]

• What is the actual behavior/output?
  type is reported as Union[int, str]
• What is the behavior/output you expect?
  type of a can only be int given the return type signature
• What are the versions of mypy and Python you are using? 7.20
  Do you see the same issue after installing mypy from Git master? yes
• What are the mypy flags you are using? (For example --strict-optional)
  python_version=3.7

I'm assuming this would be a difficult change, but I figured I'd open an issue for the sake of having a record, and in the meantime I can use assert isinstance()

Start with the minimal Python reproduction in the issue and verify the current reveal_type output under the stated Python 3.7 configuration. The change would be complete when narrowing on b as int also narrows a to int, while preserving the corresponding str case and existing behavior for unrelated unions.