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