FYI I am also going to allow Any as labelvalue types as we cast them to str anyway: #755.

Perhaps for now we should just leave out type checking on the registry until a Protocol or abc is created?

I think it's fair enough to leave it as is (with the type annotations) — if people like us are so keen on doing dubious things then a cast is OK to work around the problem in the meantime (it's no worse than it being Any — but for people who are respecting the annotations, it provides a little bit more safety net).

FYI I am also going to allow Any as labelvalue types as we cast them to str anyway

Thanks! I noticed they get cast to str internally but it's one of those things where you shouldn't second-guess the type annotations too much (though I suppose at this point, people are relying on the casts being there). It's good to have it stated explicitly, though — thanks for that (and for letting me know).

I've updated the PR to do the same for Enum :).

That sounds good, thanks!