Generalizing protocol conformance synthesis is an interesting problem, but I don't think type-level metaprogramming is the way to do it. It's a path of sadness that'll lead only to bad compiler errors, slow compile times, heavy memory usage from deeply nested type metadata, and programmer frustration because the type system is not, and really should not, be expressive enough for the sorts of things people will want to do in their protocol implementations. I also fear that we're working on too many different ways of expressing the same thing—there's already a well-developed proposal for iterating through the structure of a type with key paths, for instance. Type specialization is really just a special case of compile-time evaluation, and Swift is well set up to be able to lift values between type and value level, and also constant-fold and evaluate well-designed reflection APIs at compile time so that default implementations can be written in terms of reflection and still generate optimized specialized implementation. I think that's a far more promising direction.