os.waitid can return None

[gotyaoi]

Specifically, it can return None when its options argument contains os.WNOHANG.

If WNOHANG is specified and there are no matching children in the requested state, None is returned.

I'm not sure there's any way to express that with overrides, given that it depends on the value of a specific argument. I'm also not sure of the ergonomic effect of making it return os.waitid_result | None.

[Akuli]

Since there are 5 flags that can be bitwise-ORed, there are 16 combinations of flags where WNOHANG is set and 16 more where it isn't set. We could add overloads with unions that have 16 members, or just return waitid_result | Any, similarly to how regexes work (see e.g. #10526).

I would prefer the simpler waitid_result | Any solution, because os.waitid() is not exactly the most commonly used function in the stdlib. Usually you let subprocess.run() or similar do the waiting or you.

[JelleZijlstra]

If it's rarely used, maybe it's better to go the more type-safe route and use waitid_result | None as the annotation, forcing users to think about the possibility that it may return None.

[gotyaoi]

While part of me wants it to infer the return type, a more realistic part of me thinks that being forced to check for None is fine.