Can't infer T where Optional[T] is Optional[MyUnion]

• Are you reporting a bug, or opening a feature request?

Bug

• 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 Optional, Union, TypeVar

MyUnion = Union[int, str]
T = TypeVar("T")


def not_none(x: Optional[T]) -> T:
    assert x is not None
    return x


# Works
def f(i: Optional[int]) -> int:
    return not_none(i)


# Doesn't work
def g(u: Optional[MyUnion]) -> MyUnion:
    return not_none(u)

• What is the actual behavior/output?

a.py:19: error: Incompatible return value type (got "object", expected "Union[int, str]")

• What is the behavior/output you expect?

No error.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

mypy --version is 0.710+dev.70a4c9858fb3e736dbf4bc6cc9243c5d49bf2e5a
py --version is Python 3.7.2
Still see the issue after installing from master.

• What are the mypy flags you are using? (For example --strict-optional)

None

Start by running mypy on the minimal repro in a.py and confirm that f() succeeds while g() reports object instead of Union[int, str]. Trace the type inference for not_none(Optional[MyUnion]); done means the repro produces no error while preserving the expected return type.