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// PR c++/114138
// { dg-do compile { target c++23 } }
namespace std {
template <class T>
T&& declval() noexcept requires true;
template <class>
void declval() noexcept;
namespace detail {
struct none_such;
template <class>
using none_such_t = none_such;
template <class T>
extern const none_such_t<T> _getter_for;
template <class T>
using _decay_t = decltype(auto(declval<T>()));
static_assert(__is_same_as(_decay_t<void>, void));
}
template <const auto& Fn, class... Args>
using _result_of_t = decltype(Fn(declval<Args>()...));
template <unsigned I, class Tuple>
using tuple_element_t = _result_of_t<detail::_getter_for<detail::_decay_t<Tuple>>, char(*)[I+1], Tuple>;
template <class First, class Second>
struct pair {
First first;
Second second;
};
template <class>
inline constexpr bool _is_pair = false;
template <class First, class Second>
inline constexpr bool _is_pair<pair<First, Second>> = true;
template <class T>
concept Pair = _is_pair<decltype(auto(std::declval<T>()))>;
template <unsigned I, Pair P>
requires (I <= 1)
decltype(auto) get(P&& p) noexcept {
if constexpr (I == 0) {
return (static_cast<P&&>(p).first);
} else {
return (static_cast<P&&>(p).second);
}
}
namespace detail {
inline constexpr auto _pair_getter =
[]<unsigned J, class Pair>(char(*)[J], Pair&& p) noexcept -> decltype(auto) {
return std::get<J-1>(static_cast<Pair&&>(p));
};
template <class First, class Second>
inline constexpr auto _getter_for<pair<First, Second>> = _pair_getter;
}
}
static_assert(__is_same_as(int&, std::tuple_element_t<0, std::pair<int, float>&>));
static_assert(__is_same_as(float&&, std::tuple_element_t<1, std::pair<int, float>&&>));
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