Incorrect Undefined Variable for Outer Scope Var

[hmc-cs-mdrissi]

Describe the bug
Variable marked as undefined when coming from an outer scope and then assigned in one branch.

To Reproduce

from typing import Callable, Optional


def f(x: Optional[bool]) -> Callable[[], bool]:
    def g() -> bool:
        if not x:
            x = True
        return x

    return g

Error message, "x" is not defined for the line if not x
Expected behavior
No error.

VS Code extension or command-line
pylance 2021.5.4-pre.1

[JelleZijlstra]

That error is correct. The code will throw an UnboundLocalError at runtime. To fix it, you need to write nonlocal x.

[hmc-cs-mdrissi]

That code is simplified part of a unit test that runs successfully. Also the error message goes away if I tweak to

from typing import Callable, Optional

def f(x: Optional[bool]) -> Callable[[], bool]:
    def g() -> bool:
        if not x:
            x_ = True
        else:
          x_ = False
        return x_

    return g

even though that code starts the exact same way.

[JelleZijlstra]

That code is very different, because it creates a separate variable x_ in the inner scope.

[hmc-cs-mdrissi]

Testing that, that does explain the error.A bit surprised that even though the two examples start the same, the later mutation in first breaks the first access. Reading the nonlocal documentation explains it.