constexpr reference::reference(const reference& x) noexcept;
constexpr reference::~reference();
constexpr reference& reference::operator=(bool x) noexcept;
constexpr reference& reference::operator=(const reference& x) noexcept;
constexpr const reference& reference::operator=(bool x) const noexcept;
constexpr void swap(reference x, reference y) noexcept;
constexpr void swap(reference x, bool& y) noexcept;
constexpr void swap(bool& x, reference y) noexcept;
constexpr reference& reference::flip() noexcept;