gh-104050: Argument clinic: enable mypy's --warn-return-any setting

[AlexWaygood]

Mypy's --warn-return-any check flags when a function is declared to return a specific type, but mypy can't verify whether it's actually returning that type or not -- it can only infer a vague, unsafe Any type as the returned type. This can often lead to bugs slipping beneath the radar.

In the case of argument clinic, the check only flags a single function. But, turns out that it's a true positive! The function is incorrectly annotated at the moment -- the annotation says that it returns a FunctionType, but that's not true. The function creates a FunctionTypeinstancefn, and then returns whatever calling fn returns returns. Therefore, the proper return annotation for this function is -> Any, not -> FunctionType.

Closes#104050!

• Issue: Add type annotations to clinic.py #104050

[erlend-aasland]

Good sleuthing!

[erlend-aasland]

Should we add a comment as to why Any is the correct annotation here?

[AlexWaygood]

You weren't going to make finally closing that issue easy, were you? ;)

Let me know if a5103be is too verbose...!

[AlexWaygood]

Hmm, actually, now that I look at it again, the comment feels like it duplicates the docstring a little bit... not sure if it's worth it? I'll defer to you on whether it's helpful or not!

[erlend-aasland]

Sorry, you're right. The docstring should be enough!

[AlexWaygood]

Okay -- removed the comment again and tweaked the docstring slightly! How's it look now?

[erlend-aasland]

Yay!