I feel that compile-time evaluation will also be important to having a satisfying model for working with variadics, since it is natural to want to work with them as collections rather than use otherwise unidiomatic cons-walking techniques. That would also require figuring out the interactions between type-level and value-level programming necessary to make that work. But Swift already has semantically equivalent runtime and compile-time type system implementations, as well as operations like existential wrapping and opening that move information from type- to value-level and back, so I think we're in a good position to explore that direction.
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