NotImplemented should be implicitly a valid return value of operator methods such as __lt__, __add__, but it should be flagged as an error to return it elsewhere (unless, maybe, the return type includes a suitable type such as typing.NotImplementedType).

Currently NotImplemented has type Any which allows it to be used in any context, which is wrong.

This should be fine:

def __lt__(self, x: int) -> bool:
    if isinstance(x, int):
        return ...
    return NotImplemented

This should not be fine:

def f() -> bool:
    return NotImplemented