Use `TypeForm` in __class_getitem__ for generics?

[Andrew5057]

Now that TypeForm is in 3.15+ and typing_extensions, would it be reasonable to start using it in the __class_getitem__ stubs for generic classes? That way, we could reject constructs like list[3] without triggering false positives on things that are valid type expressions but not types.

[JelleZijlstra]

Worth trying, I think.

Type checkers might not fully support TypeForm yet, but if they don't the type should generally degrade to Any, which is no worse than the current situation.

[Andrew5057]

mypy is actually throwing errors without --enable-incomplete-feature=TypeForm, so seems like we should hold off for a bit. I might make a draft PR though.

[jonathandung]

Maybe provisionally add that in the command in the run_mypy_as_subprocess() body here.

Not sure if that still applies since typeshed now runs tests on more type checkers.