Just out of curiosity, instead of making packs be a special new concept, have you considered making them be a generic type that is known to be a tuple? Just like we have "class constraints" on generic types, we could have "tuple constraints", and tuple constrained generic parameters could have accessors on them for getting the tuple length etc.
This could allow a more dynamic model for variadics that might fit better into the overarching swift design than the static C++ model.
-Chris