eonix/gblibstdc++/include/bits/compressed_pair

#ifndef __GBLIBCPP_BITS_COMPRESSED_PAIR__
#define __GBLIBCPP_BITS_COMPRESSED_PAIR__

#include <utility>

namespace std::impl {

struct default_construct_t { };

template <typename T, int, bool is_empty = __is_empty(T)>
struct compressed_pair_element
{
    constexpr compressed_pair_element() = default;
    constexpr compressed_pair_element(const compressed_pair_element&) = default;
    constexpr compressed_pair_element(compressed_pair_element&&) = default;
    constexpr compressed_pair_element& operator=(const compressed_pair_element&) = default;
    constexpr compressed_pair_element& operator=(compressed_pair_element&&) = default;

    constexpr compressed_pair_element(default_construct_t): value{} { }
    constexpr compressed_pair_element(const T& x): value{x} { }
    constexpr compressed_pair_element(T&& x): value{std::move(x)} { }

    constexpr T& get() noexcept { return value; }
    constexpr const T& get() const noexcept { return value; }

    T value;
};

template <typename T, int N>
struct compressed_pair_element<T, N, true> : private T
{
    constexpr compressed_pair_element() = default;
    constexpr compressed_pair_element(const compressed_pair_element&) = default;
    constexpr compressed_pair_element(compressed_pair_element&&) = default;
    constexpr compressed_pair_element& operator=(const compressed_pair_element&) = default;
    constexpr compressed_pair_element& operator=(compressed_pair_element&&) = default;

    constexpr compressed_pair_element(default_construct_t): T{} { }
    constexpr compressed_pair_element(const T& x): T{x} { }
    constexpr compressed_pair_element(T&& x): T{std::move(x)} { }

    constexpr T& get() noexcept { return *this; }
    constexpr const T& get() const noexcept { return *this; }
};

template <typename T1, typename T2>
struct compressed_pair
    : private compressed_pair_element<T1, 0>, private compressed_pair_element<T2, 1> {
    using Base1 = compressed_pair_element<T1, 0>;
    using Base2 = compressed_pair_element<T2, 1>;

    constexpr compressed_pair() = default;
    constexpr compressed_pair(const compressed_pair&) = default;
    constexpr compressed_pair(compressed_pair&&) = default;
    constexpr compressed_pair& operator=(const compressed_pair&) = default;
    constexpr compressed_pair& operator=(compressed_pair&&) = default;

    constexpr compressed_pair(default_construct_t): Base1{default_construct_t{}}, Base2{default_construct_t{}} { }
    constexpr compressed_pair(const T1& x, const T2& y): Base1{x}, Base2{y} { }
    constexpr compressed_pair(T1&& x, T2&& y): Base1{std::move(x)}, Base2{std::move(y)} { }

    constexpr T1& first() noexcept { return Base1::get(); }
    constexpr const T1& first() const noexcept { return Base1::get(); }
    constexpr T2& second() noexcept { return Base2::get(); }
    constexpr const T2& second() const noexcept { return Base2::get(); }
};

} // namespace std::impl

#endif