template<class T = void> struct negate {
  constexpr T operator()(const T& x) const;
};
constexpr T operator()(const T& x) const;
template<> struct negate<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));