Flag `None` check on value that cannot be `None`

[nickdrozd]

Say I have function that maybe returns None and I check the result:

def f() -> int | None: ...

if f() is None: ...

I rewrite the function so that it never returns None:

def f() -> int: ...

if f() is None: ...

Now the is None check makes no sense -- it will always fail. This is a mistake introduced in refactoring and it should be flagged. (And similar for is not None checks)

In Rust I don't think this kind of error is possible:

fn f() -> Option<u8>;

if f().is_none() {}  // okay

fn g() -> u8;

if g().is_none() {} // no good

[A5rocks]

Both --warn-unreachable and --strict-equality should work for this. But they don't! That's a bug.

Maybe worth closing this to make a bug report? Or someone can change tags (I can't :'( ).

[nickdrozd]

Forgot to add, this affects Mypy 1.14.1 with Python 3.14, using --strict setting. (Not sure if --warn-unreachable is included in that, but if it isn't it should be.)

Switching the tag from feature to bug is fine with me :)

[A5rocks]

It isn't included in that. Maybe thumbs up #18078!

[nickdrozd]

Here is a full example:

from random import randrange

def f(x: int) -> int | None:
    return None if x == 0 else 2 * x

def g(x: int) -> int:
    return 2 * x

if __name__ == '__main__':
    if f(randrange(10)) is None:
        print('f: got 0')

    if g(randrange(10)) is None:  # <-- unreachable but not flagged
        print('g: got 0')

mypy --strict --warn-unreachable does not raise any warnings, but it should. So this is a FALSE NEGATIVE bug. Or a new feature, whatever, but in any case this situation should be flagged.