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;
};