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Can't infer T where Optional[T] is Optional[MyUnion] #6751

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@ghost

Description

  • 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

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