template <class T1, class T2>
struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };
tuple_element<0, pair<T1, T2>>::type
tuple_element<1, pair<T1, T2>>::type
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;
Returns: If I == 0 returns a reference to p.first; if I == 1 returns a reference to p.second; otherwise the program is ill-formed.
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;