mypy forgets that value is not None inside function definition

[schuelermine]

Bug Report

mypy thinks that the type of a variable is Optional[A] when it has been previously determined to not be None when the variable is used in a function definition

To Reproduce

from typing import TypeVar, ParamSpec, Concatenate
from collections.abc import Callable

A = TypeVar("A")
B = ParamSpec("B")
C = TypeVar("C")


def guard_none(a: A | None, f: Callable[Concatenate[A, B], C]) -> Callable[B, C]:
    if a is None:
        raise TypeError()
    else:
        
        def g(*args: B.args, **kwargs: B.kwargs) -> C:
            return f(a, *args, **kwargs)
        
        return g

Expected Behavior

I expect no error.

Actual Behavior

mypy produces this error:

main.py:15: error: Argument 1 has incompatible type "Optional[A]"; expected "A"  [arg-type]

Your Environment

mypy playground

• Mypy version used: 1.2.0
• Mypy command-line flags: --strict
• Python version used: 3.11

[schuelermine]

apparently this is a duplicate

[JelleZijlstra]

Of #2608