Right now there's an uncovered branch in TypeVarTuple.__typing_prepare_subst__:

And it looks like there's a bug in it, these two lines:

 for param in enumerate(params[typevartuple_index + 1:]): if isinstance(param, TypeVarTuple):

This if's body will never be executed, because enumerate always returns tuple[int, T], it is never TypeVarTuple.

So, I think that we need to remove enumerate() call here and add a test like:

>>> from typing import Generic, TypeVarTuple >>> V = TypeVarTuple('V') >>> T = TypeVarTuple('T') >>> class My(Generic[*T]): pass ... >>> My[*T, *V][*V] TypeError: More than one TypeVarTuple parameter in __main__.My[*T, *V]

I will send a PR 😉