template<class... Types>
constexpr bool operator==(const variant<Types...>& v, const variant<Types...>& w);
template<class... Types>
constexpr bool operator!=(const variant<Types...>& v, const variant<Types...>& w);
template<class... Types>
constexpr bool operator<(const variant<Types...>& v, const variant<Types...>& w);
template<class... Types>
constexpr bool operator>(const variant<Types...>& v, const variant<Types...>& w);
template<class... Types>
constexpr bool operator<=(const variant<Types...>& v, const variant<Types...>& w);
template<class... Types>
constexpr bool operator>=(const variant<Types...>& v, const variant<Types...>& w);