> I think Swift maybe does something more clever with witness tables, but I don't recally exactly how it works.

Pestov actually wrote a long explanation of what it is that Swift does there[1,2]. And I’m almost sure you’ve already seen it, but it’s been on my reading list forever and I’m hoping that maybe if I can’t get myself to read it, than somebody else will see this comment, get interested and do it.

[1] http://download.swift.org/docs/assets/generics.pdf

[2] https://github.com/apple/swift/tree/main/docs/Generics