http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/... Specifies that the T... are placeholders; however, it doesn't specify that they be unique; hence, tuple<concept,_a,_a> x(int(0),std::vector<int>(0)); would satisfy the current description but wouldn't be right. Of course no one would intentionally do that; however, if the Tags are calculated in some meta-program, it might happened accidentally. Hence the requires class should also say, "If T... are not unique, no diagnostic occurs", or something similar. The reason I think "no diagnostic occurs" is that I've looked briefly in tuple.hpp, at the: ::boost::type_erasure::cons<Concept, T...> impl; in the class tuple definition and the cons definition, and I don't see any check for duplicates. -regards, Larry