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