20 General utilities library [utilities]

20.5 Compile-time integer sequences [intseq]

20.5.2 Class template integer_sequence [intseq.intseq]

namespace std {
  template<class T, T... I>
  struct integer_sequence {
    typedef T value_type;
    static constexpr size_t size() noexcept { return sizeof...(I); }
  };
}

T shall be an integer type.