Checking if a class has __bool__ implemented to return a non-boolean (or NoReturn) and a conditional is used on that class

[Dr-Irv]

Consider the following code that raises an exception:

from typing_extensions import NoReturn


class Foo:
    def __init__(self, x: int):
        self._x = x

    def __bool__(self) -> NoReturn:
        raise Exception("bad idea")


y = Foo(10)

if y:
    print("should not happen if True")
else:
    print("should not happen if False")

In this case, we would like the type checker to report that if y is invalid, because __bool__() returns NoReturn

This came up here: microsoft/pyright#5039

In that discussion, Eric Traut, author of pyright wrote:

I think there's a good argument to be made that conditionals in Python don't use the result of bool in a covariant manner. They require a bool response and generate an exception if bool returns any other type. In other words, they treat the result as invariant, not covariant. That would imply that if bool is annotated with a return type of anything other than bool, it should be treated as an error when used in a conditional.

He has changed pyright to recognize this. He asked that it also be discussed here to see if other type checkers should do the same.

Note - this pattern is used in pandas as the result of testing the truth of a pandas DataFrame or Series is ambiguous, so pandas raises an Exception