generic type assertion is incompatible with generic sorted key

Bug Report

I have a function that removes the nullability of a value, so it takes T | None and returns T. This function doesn't seem to work in functions that have overloaded generic function args, like sorted.

If I create my own sorted function without an overload, this issue doesn't occur.

def my_sorted(__iterable: Iterable[_T], *, key: Callable[[_T], SupportsLessThan], reverse: bool = ...) -> List[_T]: ...

To Reproduce

from __future__ import annotations

from typing import TypeVar

T = TypeVar("T")


def unwrap(val: T | None) -> T:
    assert val is not None
    return val


x: list[str | None]
sorted(x, key=lambda x: unwrap(x).upper())
#                       <nothing> has no attribute "upper"  [attr-defined]

Expected Behavior
unwrap(x) should return str.

Actual Behavior

unwrap(x) returns <nothing>.

Your Environment

• Mypy version used: 0.800

Start by reproducing the example with mypy 0.800 and inspect the overloaded sorted signature in the linked typeshed builtins.pyi. Done means the key lambda accepts str | None input, unwrap(x) is inferred as str, and the upper attribute error disappears.