I think this is the last class that can be usefully converted to become a dataclass, FTR. For the remaining classes in argument clinic, ISTM converting the classes to become dataclasses would make the code more complicated, rather than improving readability. And that would defeat the object.

• Convert the class into a dataclass (this necessitates moving is_a_simple_defined into the global namespace)

Makes more sense to have it there as a global, IMO