Ok, https://github.com/python/cpython/pull/32341/files is a reference of how the current implementation behaves. Fwiw, it *is* mostly correct - with a few minor tweaks it might be alright for at least the 3.11 release.

In particular, instead of dealing with the thorny issue of what to do about splitting unpacked arbitrary-length tuples over multiple type variables - e.g. C[T, *Ts][*tuple[int, ...]] - instead either deciding to try and evaluate it properly and living with the complexity, or leaving it unsimplified and living with the __args__, __parameters__ and __origin__ problem - for now, we could just raise an exception for any substitutions which involve an unpacked arbitrary-length tuple, since I'd guess it's going to be an extremely rare use-case.