Since you asked what I think,
"pseudo-signatures > usage patterns"
but that's probably not the kind of information you were looking for :-)
Yes, that's more of a critique to N3351 than my design to implement it but it's a fare comment. If N3351 approach is to be abandoned in favor of pseudo-signatures then no point in implementing N3351 but I don't think there's consensus on this either way yet. I personally don't have an opinion yet... (and maybe I should have an opinion before I go off an impl the entire N3351).
Even if you decide to go with pseudo-signatures, one novel thing from N3351 is worth adapting: treating every concept as a predicate, so that the programmer can check himself whether a certain set of types models the concept: static_assert( ForwardIterator<int*>, "ouch" ); and so that you can combine concept requirements with any boolean meta-function: template <class T> requires Copyable<T> && sizeof(T) < 8 ... Regards, &rzej