21 Metaprogramming library [meta]
21.2 Compile-time integer sequences [intseq]
namespace std {
  template<class T, T... I> struct integer_sequence {
    using value_type = T;
    static constexpr size_t size() noexcept { return sizeof...(I); }
  };
}
 Mandates: 
T is an integer type
.