16 Language support library [language.support]

namespace std {
  // [cmp.categories], comparison category types
  class weak_equality;
  class strong_equality;
  class partial_ordering;
  class weak_ordering;
  class strong_ordering;

  // named comparison functions
  constexpr bool is_eq  (weak_equality cmp) noexcept    { return cmp == 0; }
  constexpr bool is_neq (weak_equality cmp) noexcept    { return cmp != 0; }
  constexpr bool is_lt  (partial_ordering cmp) noexcept { return cmp < 0; }
  constexpr bool is_lteq(partial_ordering cmp) noexcept { return cmp <= 0; }
  constexpr bool is_gt  (partial_ordering cmp) noexcept { return cmp > 0; }
  constexpr bool is_gteq(partial_ordering cmp) noexcept { return cmp >= 0; }

  // [cmp.common], common comparison category type
  template<class... Ts>
  struct common_comparison_category {
    using type = see below;
  };
  template<class... Ts>
    using common_comparison_category_t = typename common_comparison_category<Ts...>::type;

  // [cmp.alg], comparison algorithms
  template<class T> constexpr strong_ordering strong_order(const T& a, const T& b);
  template<class T> constexpr weak_ordering weak_order(const T& a, const T& b);
  template<class T> constexpr partial_ordering partial_order(const T& a, const T& b);
  template<class T> constexpr strong_equality strong_equal(const T& a, const T& b);
  template<class T> constexpr weak_equality weak_equal(const T& a, const T& b);
}