template<class Err = E>
constexpr explicit unexpected(Err&& e);
template<class... Args>
constexpr explicit unexpected(in_place_t, Args&&... args);
template<class U, class... Args>
constexpr explicit unexpected(in_place_t, initializer_list<U> il, Args&&... args);
constexpr const E& error() const & noexcept;
constexpr E& error() & noexcept;
constexpr E&& error() && noexcept;
constexpr const E&& error() const && noexcept;
constexpr void swap(unexpected& other) noexcept(is_nothrow_swappable_v<E>);
friend constexpr void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y)));