template<class T = void> struct logical_not {
  constexpr bool operator()(const T& x) const;
};
constexpr bool operator()(const T& x) const;
template<> struct logical_not<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));
  using is_transparent = unspecified;
};
template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));