template<class T1, class T2>
struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };
template<size_t I, class T1, class T2>
struct tuple_element<I, pair<T1, T2>> {
using type = see below ;
};
template<size_t I, class T1, class T2>
constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>& p) noexcept;
template<size_t I, class T1, class T2>
constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>& p) noexcept;
template<size_t I, class T1, class T2>
constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&& p) noexcept;
template<size_t I, class T1, class T2>
constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&& p) noexcept;
template<class T1, class T2>
constexpr T1& get(pair<T1, T2>& p) noexcept;
template<class T1, class T2>
constexpr const T1& get(const pair<T1, T2>& p) noexcept;
template<class T1, class T2>
constexpr T1&& get(pair<T1, T2>&& p) noexcept;
template<class T1, class T2>
constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;
template<class T2, class T1>
constexpr T2& get(pair<T1, T2>& p) noexcept;
template<class T2, class T1>
constexpr const T2& get(const pair<T1, T2>& p) noexcept;
template<class T2, class T1>
constexpr T2&& get(pair<T1, T2>&& p) noexcept;
template<class T2, class T1>
constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;