`--warn-unreachable` doesn't flag unnecessary comparison to `None`

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.

Originally posted by @nickdrozd in #18386

This might be intentional, but I think it's counterintuitive and I can't really see a benefit. Making an issue to make sure I get around to investigating this.

首先，使用 mypy --strict --warn-unreachable 重现提供的示例，并确认涉及 g 的比较不会被报告。跟踪针对与 None 进行比较的 --warn-unreachable 分析；当不必要的比较被诊断出来，同时没有错误地标记涉及 f 的可达比较时，即表示完成。