In a somewhat misspent youth, I was a grad student of Jaakko Järvi when he and the rest of the Indiana folk were pitching variadic templates and concepts. I was browbeat into attempting to add concept constrained variadic templates to GCC. So... it turns out, for this problem, for precise constraints, the number of constraints is strongly exponential (factorial of factorials). I think we ended up going with the intersection of all the concepts, instead; which totally sucked & made constrained variadic templates pretty useless.

I suppose it doesn't matter since the concepts proposal was torpedoed. (But that's another story.)