115     struct _Tuple_impl; 
118     struct _Tuple_impl<_Idx>  struct
120       template<std::size_t, typename...> friend class _Tuple_impl;
122       _Tuple_impl() = default;
129     struct _Tuple_impl<_Idx, _Head, _Tail...>  struct
130     : public _Tuple_impl<_Idx + 1, _Tail...>,
134       template<std::size_t, typename...> friend class _Tuple_impl;
136       typedef _Tuple_impl<_Idx + 1, _Tail...> _Inherited;  argument
140       _M_head(const _Tuple_impl& __t) noexcept { return _Base::_M_head(__t); }  in _M_head()  argument
143       _M_tail(const _Tuple_impl& __t) noexcept { return __t; }  in _M_tail()  argument
147         constexpr _Tuple_impl(_UHead&& __head, _UTail&&... __tail)  in _Tuple_impl()  argument
152       _Tuple_impl(_Tuple_impl&& __in)  in _Tuple_impl()  argument
161     class tuple : public _Tuple_impl<0, _Elements...>
163       typedef _Tuple_impl<0, _Elements...> _Inherited;
203     __get_helper(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept  in __get_helper()
204     { return _Tuple_impl<__i, _Head, _Tail...>::_M_head(__t); }  in __get_helper()