22 General utilities library [utilities]
template<class T = void> struct less {
  constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
template<> struct less<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));
  using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));
Returns: 
std::forward<T>(t) < std::forward<U>(u).