constexpr const T* operator->() const noexcept;
constexpr T* operator->() noexcept;
constexpr const T& operator*() const & noexcept;
constexpr T& operator*() & noexcept;
constexpr T&& operator*() && noexcept;
constexpr const T&& operator*() const && noexcept;
constexpr explicit operator bool() const noexcept;
constexpr bool has_value() const noexcept;
constexpr const T& value() const &;
constexpr T& value() &;
constexpr T&& value() &&;
constexpr const T&& value() const &&;
template<class U> constexpr T value_or(U&& v) const &;
template<class U> constexpr T value_or(U&& v) &&;