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#ifndef _GLIBCXX_OPTIONAL
#define _GLIBCXX_OPTIONAL 1

#if __cplusplus >= 201703L

#include <utility>
#include <type_traits>
#include <exception>
#include <new>
#include <initializer_list>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/enable_special_members.h>
#include <bits/stl_construct.h>
#if __cplusplus > 201703L
# include <compare>
#endif

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
 *  @addtogroup utilities
 *  @{
 */

#define __cpp_lib_optional 201606L

template<typename _Tp>
class optional;

/// Tag type to disengage optional objects.
struct nullopt_t {
    // Do not user-declare default constructor at all for
    // optional_value = {} syntax to work.
    // nullopt_t() = delete;

// Used for constructing nullopt.
    enum class _Construct { _Token };

// Must be constexpr for nullopt_t to be literal.
    explicit constexpr nullopt_t(_Construct) { }
};

/// Tag to disengage optional objects.
inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };

/**
 *  @brief Exception class thrown when a disengaged optional object is
 *  dereferenced.
 *  @ingroup exceptions
 */
class bad_optional_access : public exception
{
public:
    bad_optional_access() { }

virtual const char* what() const noexcept override
    {
        return "bad optional access";
    }

virtual ~bad_optional_access() noexcept = default;
};

void
__throw_bad_optional_access()
__attribute__((__noreturn__));

// XXX Does not belong here.
inline void
__throw_bad_optional_access() {
    _GLIBCXX_THROW_OR_ABORT(bad_optional_access());
}

// This class template manages construction/destruction of
// the contained value for a std::optional.
template <typename _Tp>
struct _Optional_payload_base {
    using _Stored_type = remove_const_t<_Tp>;

_Optional_payload_base() = default;
    ~_Optional_payload_base() = default;

template<typename... _Args>
    constexpr
    _Optional_payload_base(in_place_t __tag, _Args&&... __args)
        : _M_payload(__tag, std::forward<_Args>(__args)...),
          _M_engaged(true)
    { }

template<typename _Up, typename... _Args>
    constexpr
    _Optional_payload_base(std::initializer_list<_Up> __il,
                           _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true)
    { }

// Constructor used by _Optional_base copy constructor when the
    // contained value is not trivially copy constructible.
    constexpr
    _Optional_payload_base(bool __engaged,
                           const _Optional_payload_base& __other)
    {
        if (__other._M_engaged) {
            this->_M_construct(__other._M_get());
        }
    }

// Constructor used by _Optional_base move constructor when the
    // contained value is not trivially move constructible.
    constexpr
    _Optional_payload_base(bool __engaged,
                           _Optional_payload_base&& __other)
    {
        if (__other._M_engaged) {
            this->_M_construct(std::move(__other._M_get()));
        }
    }

// Copy constructor is only used to when the contained value is
    // trivially copy constructible.
    _Optional_payload_base(const _Optional_payload_base&) = default;

// Move constructor is only used to when the contained value is
    // trivially copy constructible.
    _Optional_payload_base(_Optional_payload_base&&) = default;

_Optional_payload_base&
    operator=(const _Optional_payload_base&) = default;

_Optional_payload_base&
    operator=(_Optional_payload_base&&) = default;

// used to perform non-trivial copy assignment.
    constexpr void
    _M_copy_assign(const _Optional_payload_base& __other)
    {
        if (this->_M_engaged && __other._M_engaged) {
            this->_M_get() = __other._M_get();
        } else {
            if (__other._M_engaged) {
                this->_M_construct(__other._M_get());
            } else {
                this->_M_reset();
            }
        }
    }

// used to perform non-trivial move assignment.
    constexpr void
    _M_move_assign(_Optional_payload_base&& __other)
    noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
             is_nothrow_move_assignable<_Tp>>)
    {
        if (this->_M_engaged && __other._M_engaged) {
            this->_M_get() = std::move(__other._M_get());
        } else {
            if (__other._M_engaged) {
                this->_M_construct(std::move(__other._M_get()));
            } else {
                this->_M_reset();
            }
        }
    }

struct _Empty_byte { };

template<typename _Up, bool = is_trivially_destructible_v<_Up>>
    union _Storage {
        constexpr _Storage() noexcept : _M_empty() { }

template<typename... _Args>
        constexpr
        _Storage(in_place_t, _Args&&... __args)
            : _M_value(std::forward<_Args>(__args)...)
        { }

template<typename _Vp, typename... _Args>
        constexpr
        _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
            : _M_value(__il, std::forward<_Args>(__args)...)
        { }

_Empty_byte _M_empty;
        _Up _M_value;
    };

template<typename _Up>
    union _Storage<_Up, false> {
        constexpr _Storage() noexcept : _M_empty() { }

template<typename... _Args>
        constexpr
        _Storage(in_place_t, _Args&&... __args)
            : _M_value(std::forward<_Args>(__args)...)
        { }

// User-provided destructor is needed when _Up has non-trivial dtor.
        constexpr ~_Storage() { }

_Empty_byte _M_empty;
        _Up _M_value;
    };

_Storage<_Stored_type> _M_payload;

bool _M_engaged = false;

template<typename... _Args>
    constexpr void
    _M_construct(_Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
    {
        std::construct_at(std::__addressof(this->_M_payload),
            std::forward<_Args>(__args)...);
        this->_M_engaged = true;
    }

constexpr void
    _M_destroy() noexcept
    {
        _M_engaged = false;
        _M_payload._M_value.~_Stored_type();
    }

// The _M_get() operations have _M_engaged as a precondition.
    // They exist to access the contained value with the appropriate
    // const-qualification, because _M_payload has had the const removed.

constexpr _Tp&
    _M_get() noexcept
    {
        return this->_M_payload._M_value;
    }

constexpr const _Tp&
    _M_get() const noexcept
    {
        return this->_M_payload._M_value;
    }

// _M_reset is a 'safe' operation with no precondition.
    constexpr void
    _M_reset() noexcept
    {
        if (this->_M_engaged) {
            _M_destroy();
        }
    }
};

// Class template that manages the payload for optionals.
template <typename _Tp,
          bool /*_HasTrivialDestructor*/ =
          is_trivially_destructible_v<_Tp>,
          bool /*_HasTrivialCopy */ =
          is_trivially_copy_assignable_v<_Tp>
          && is_trivially_copy_constructible_v<_Tp>,
          bool /*_HasTrivialMove */ =
          is_trivially_move_assignable_v<_Tp>
          && is_trivially_move_constructible_v<_Tp>>
struct _Optional_payload;

// Payload for potentially-constexpr optionals (trivial copy/move/destroy).
template <typename _Tp>
struct _Optional_payload<_Tp, true, true, true>
    : _Optional_payload_base<_Tp> {
    using _Optional_payload_base<_Tp>::_Optional_payload_base;

_Optional_payload() = default;
};

// Payload for optionals with non-trivial copy construction/assignment.
template <typename _Tp>
struct _Optional_payload<_Tp, true, false, true>
    : _Optional_payload_base<_Tp> {
    using _Optional_payload_base<_Tp>::_Optional_payload_base;

_Optional_payload() = default;
    ~_Optional_payload() = default;
    _Optional_payload(const _Optional_payload&) = default;
    _Optional_payload(_Optional_payload&&) = default;
    _Optional_payload& operator=(_Optional_payload&&) = default;

// Non-trivial copy assignment.
    constexpr
    _Optional_payload&
    operator=(const _Optional_payload& __other)
    {
        this->_M_copy_assign(__other);
        return *this;
    }
};

// Payload for optionals with non-trivial move construction/assignment.
template <typename _Tp>
struct _Optional_payload<_Tp, true, true, false>
    : _Optional_payload_base<_Tp> {
    using _Optional_payload_base<_Tp>::_Optional_payload_base;

// Non-trivial move assignment.
    constexpr
    _Optional_payload&
    operator=(_Optional_payload&& __other)
    noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
             is_nothrow_move_assignable<_Tp>>)
    {
        this->_M_move_assign(std::move(__other));
        return *this;
    }
};

// Payload for optionals with non-trivial copy and move assignment.
template <typename _Tp>
struct _Optional_payload<_Tp, true, false, false>
    : _Optional_payload_base<_Tp> {
    using _Optional_payload_base<_Tp>::_Optional_payload_base;

_Optional_payload() = default;
    ~_Optional_payload() = default;
    _Optional_payload(const _Optional_payload&) = default;
    _Optional_payload(_Optional_payload&&) = default;

// Non-trivial copy assignment.
    constexpr
    _Optional_payload&
    operator=(const _Optional_payload& __other)
    {
        this->_M_copy_assign(__other);
        return *this;
    }

// Payload for optionals with non-trivial destructors.
template <typename _Tp, bool _Copy, bool _Move>
struct _Optional_payload<_Tp, false, _Copy, _Move>
    : _Optional_payload<_Tp, true, false, false> {
    // Base class implements all the constructors and assignment operators:
    using _Optional_payload<_Tp, true, false, false>::_Optional_payload;
    _Optional_payload() = default;
    _Optional_payload(const _Optional_payload&) = default;
    _Optional_payload(_Optional_payload&&) = default;
    _Optional_payload& operator=(const _Optional_payload&) = default;
    _Optional_payload& operator=(_Optional_payload&&) = default;

// Destructor needs to destroy the contained value:
    constexpr ~_Optional_payload()
    {
        this->_M_reset();
    }
};

// Common base class for _Optional_base<T> to avoid repeating these
// member functions in each specialization.
template<typename _Tp, typename _Dp>
class _Optional_base_impl
{
protected:
    using _Stored_type = remove_const_t<_Tp>;

// The _M_construct operation has !_M_engaged as a precondition
    // while _M_destruct has _M_engaged as a precondition.
    template<typename... _Args>
    constexpr void
    _M_construct(_Args&&... __args)
    noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
    {
        std::construct_at(std::__addressof(static_cast<_Dp*>(this)->_M_payload._M_payload._M_value),
            std::forward<_Args>(__args)...);
        static_cast<_Dp*>(this)->_M_payload._M_engaged = true;
    }

constexpr void
    _M_destruct() noexcept
    {
        static_cast<_Dp*>(this)->_M_payload._M_destroy();
    }

// _M_reset is a 'safe' operation with no precondition.
    constexpr void
    _M_reset() noexcept
    {
        static_cast<_Dp*>(this)->_M_payload._M_reset();
    }

constexpr bool _M_is_engaged() const noexcept
    {
        return static_cast<const _Dp*>(this)->_M_payload._M_engaged;
    }

// The _M_get operations have _M_engaged as a precondition.
    constexpr _Tp&
    _M_get() noexcept
    {
        __glibcxx_assert(this->_M_is_engaged());
        return static_cast<_Dp*>(this)->_M_payload._M_get();
    }

constexpr const _Tp&
    _M_get() const noexcept
    {
        __glibcxx_assert(this->_M_is_engaged());
        return static_cast<const _Dp*>(this)->_M_payload._M_get();
    }
};

/**
  * @brief Class template that provides copy/move constructors of optional.
  *
  * Such a separate base class template is necessary in order to
  * conditionally make copy/move constructors trivial.
  *
  * When the contained value is trivially copy/move constructible,
  * the copy/move constructors of _Optional_base will invoke the
  * trivial copy/move constructor of _Optional_payload. Otherwise,
  * they will invoke _Optional_payload(bool, const _Optional_payload&)
  * or _Optional_payload(bool, _Optional_payload&&) to initialize
  * the contained value, if copying/moving an engaged optional.
  *
  * Whether the other special members are trivial is determined by the
  * _Optional_payload<_Tp> specialization used for the _M_payload member.
  *
  * @see optional, _Enable_special_members
  */
template<typename _Tp,
         bool = is_trivially_copy_constructible_v<_Tp>,
         bool = is_trivially_move_constructible_v<_Tp>>
struct _Optional_base
    : _Optional_base_impl<_Tp, _Optional_base<_Tp>> {
    // Constructors for disengaged optionals.
    constexpr _Optional_base() = default;

// Constructors for engaged optionals.
    template<typename... _Args,
             enable_if_t<is_constructible_v<_Tp, _Args&&...>, bool> = false>
    constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(in_place,
                     std::forward<_Args>(__args)...) { }

template<typename _Up, typename... _Args,
             enable_if_t<is_constructible_v<_Tp,
                                            initializer_list<_Up>&,
                                            _Args&&...>, bool> = false>
    constexpr explicit _Optional_base(in_place_t,
                                      initializer_list<_Up> __il,
                                      _Args&&... __args)
        : _M_payload(in_place,
                     __il, std::forward<_Args>(__args)...)
    { }

// Copy and move constructors.
    constexpr _Optional_base(const _Optional_base& __other)
        : _M_payload(__other._M_payload._M_engaged,
                     __other._M_payload)
    { }

constexpr _Optional_base(_Optional_base&& __other)
    noexcept(is_nothrow_move_constructible_v<_Tp>)
        : _M_payload(__other._M_payload._M_engaged,
                     std::move(__other._M_payload))
    { }

// Assignment operators.
    _Optional_base& operator=(const _Optional_base&) = default;
    _Optional_base& operator=(_Optional_base&&) = default;

_Optional_payload<_Tp> _M_payload;
};

template<typename _Tp>
struct _Optional_base<_Tp, false, true>
    : _Optional_base_impl<_Tp, _Optional_base<_Tp>> {
    // Constructors for disengaged optionals.
    constexpr _Optional_base() = default;

// Copy and move constructors.
    constexpr _Optional_base(const _Optional_base& __other)
        : _M_payload(__other._M_payload._M_engaged,
                     __other._M_payload)
    { }

constexpr _Optional_base(_Optional_base&& __other) = default;

// Assignment operators.
    _Optional_base& operator=(const _Optional_base&) = default;
    _Optional_base& operator=(_Optional_base&&) = default;

_Optional_payload<_Tp> _M_payload;
};

template<typename _Tp>
struct _Optional_base<_Tp, true, false>
    : _Optional_base_impl<_Tp, _Optional_base<_Tp>> {
    // Constructors for disengaged optionals.
    constexpr _Optional_base() = default;

// Copy and move constructors.
    constexpr _Optional_base(const _Optional_base& __other) = default;

// Assignment operators.
    _Optional_base& operator=(const _Optional_base&) = default;
    _Optional_base& operator=(_Optional_base&&) = default;

_Optional_payload<_Tp> _M_payload;
};

template<typename _Tp>
struct _Optional_base<_Tp, true, true>
    : _Optional_base_impl<_Tp, _Optional_base<_Tp>> {
    // Constructors for disengaged optionals.
    constexpr _Optional_base() = default;

// Copy and move constructors.
    constexpr _Optional_base(const _Optional_base& __other) = default;
    constexpr _Optional_base(_Optional_base&& __other) = default;

// Assignment operators.
    _Optional_base& operator=(const _Optional_base&) = default;
    _Optional_base& operator=(_Optional_base&&) = default;

_Optional_payload<_Tp> _M_payload;
};

template<typename _Tp>
class optional;

template<typename _Tp, typename _Up>
using __converts_from_optional =
    __or_<is_constructible<_Tp, const optional<_Up>&>,
    is_constructible<_Tp, optional<_Up>&>,
    is_constructible<_Tp, const optional<_Up>&&>,
    is_constructible<_Tp, optional<_Up>&&>,
    is_convertible<const optional<_Up>&, _Tp>,
    is_convertible<optional<_Up>&, _Tp>,
    is_convertible<const optional<_Up>&&, _Tp>,
    is_convertible<optional<_Up>&&, _Tp>>;

template<typename _Tp, typename _Up>
using __assigns_from_optional =
    __or_<is_assignable<_Tp&, const optional<_Up>&>,
    is_assignable<_Tp&, optional<_Up>&>,
    is_assignable<_Tp&, const optional<_Up>&&>,
    is_assignable<_Tp&, optional<_Up>&&>>;

/**
  * @brief Class template for optional values.
  */
template<typename _Tp>
class optional
    : private _Optional_base<_Tp>,
      private _Enable_copy_move<
// Copy constructor.
      is_copy_constructible_v<_Tp>,
// Copy assignment.
      __and_v<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>,
// Move constructor.
      is_move_constructible_v<_Tp>,
// Move assignment.
      __and_v<is_move_constructible<_Tp>, is_move_assignable<_Tp>>,
// Unique tag type.
      optional<_Tp>>
{
    static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
    static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
    static_assert(!is_reference_v<_Tp>);

private:
    using _Base = _Optional_base<_Tp>;

// SFINAE helpers
    template<typename _Up>
    using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
    template<typename _Up>
    using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
    template<typename... _Cond>
    using _Requires = enable_if_t<__and_v<_Cond...>, bool>;

public:
    using value_type = _Tp;

constexpr optional() = default;

constexpr optional(nullopt_t) noexcept { }

// Converting constructors for engaged optionals.
    template<typename _Up = _Tp,
             _Requires<__not_self<_Up>, __not_tag<_Up>,
                       is_constructible<_Tp, _Up&&>,
                       is_convertible<_Up&&, _Tp>> = true>
    constexpr
    optional(_Up&& __t)
        : _Base(std::in_place, std::forward<_Up>(__t)) { }

template<typename _Up = _Tp,
             _Requires<__not_self<_Up>, __not_tag<_Up>,
                       is_constructible<_Tp, _Up&&>,
                       __not_<is_convertible<_Up&&, _Tp>>> = false>
                       explicit constexpr
                       optional(_Up&& __t)
                           : _Base(std::in_place, std::forward<_Up>(__t)) { }

template<typename _Up,
             _Requires<__not_<is_same<_Tp, _Up>>,
                       is_constructible<_Tp, const _Up&>,
                       is_convertible<const _Up&, _Tp>,
                       __not_<__converts_from_optional<_Tp, _Up>>> = true>
                       constexpr
                       optional(const optional<_Up>& __t)
    {
        if (__t) {
            emplace(*__t);
        }
    }

template<typename _Up,
             _Requires<__not_<is_same<_Tp, _Up>>,
                       is_constructible<_Tp, const _Up&>,
                       __not_<is_convertible<const _Up&, _Tp>>,
                       __not_<__converts_from_optional<_Tp, _Up>>> = false>
                       explicit constexpr
                       optional(const optional<_Up>& __t)
    {
        if (__t) {
            emplace(*__t);
        }
    }

template <typename _Up,
              _Requires<__not_<is_same<_Tp, _Up>>,
                        is_constructible<_Tp, _Up&&>,
                        is_convertible<_Up&&, _Tp>,
                        __not_<__converts_from_optional<_Tp, _Up>>> = true>
                        constexpr
                        optional(optional<_Up>&& __t)
    {
        if (__t) {
            emplace(std::move(*__t));
        }
    }

template <typename _Up,
              _Requires<__not_<is_same<_Tp, _Up>>,
                        is_constructible<_Tp, _Up&&>,
                        __not_<is_convertible<_Up&&, _Tp>>,
                        __not_<__converts_from_optional<_Tp, _Up>>> = false>
                        explicit constexpr
                        optional(optional<_Up>&& __t)
    {
        if (__t) {
            emplace(std::move(*__t));
        }
    }

template<typename... _Args,
             _Requires<is_constructible<_Tp, _Args&&...>> = false>
    explicit constexpr
    optional(in_place_t, _Args&&... __args)
        : _Base(std::in_place, std::forward<_Args>(__args)...) { }

template<typename _Up, typename... _Args,
             _Requires<is_constructible<_Tp,
                                        initializer_list<_Up>&,
                                        _Args&&...>> = false>
    explicit constexpr
    optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
        : _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }

// Assignment operators.
    constexpr optional&
    operator=(nullopt_t) noexcept
    {
        this->_M_reset();
        return *this;
    }

template<typename _Up = _Tp>
    constexpr
    enable_if_t<__and_v<__not_self<_Up>,
                        __not_<__and_<is_scalar<_Tp>,
                                      is_same<_Tp, decay_t<_Up>>>>,
                                      is_constructible<_Tp, _Up>,
                                      is_assignable<_Tp&, _Up>>,
                               optional&>
                        operator=(_Up&& __u)
    {
        if (this->_M_is_engaged()) {
            this->_M_get() = std::forward<_Up>(__u);
        } else {
            this->_M_construct(std::forward<_Up>(__u));
        }

return *this;
    }

template<typename _Up>
    constexpr
    enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
                        is_constructible<_Tp, const _Up&>,
                        is_assignable<_Tp&, _Up>,
                        __not_<__converts_from_optional<_Tp, _Up>>,
                        __not_<__assigns_from_optional<_Tp, _Up>>>,
                               optional&>
                        operator=(const optional<_Up>& __u)
    {
        if (__u) {
            if (this->_M_is_engaged()) {
                this->_M_get() = *__u;
            } else {
                this->_M_construct(*__u);
            }
        } else {
            this->_M_reset();
        }
        return *this;
    }

template<typename _Up>
    constexpr
    enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
                        is_constructible<_Tp, _Up>,
                        is_assignable<_Tp&, _Up>,
                        __not_<__converts_from_optional<_Tp, _Up>>,
                        __not_<__assigns_from_optional<_Tp, _Up>>>,
                               optional&>
                        operator=(optional<_Up>&& __u)
    {
        if (__u) {
            if (this->_M_is_engaged()) {
                this->_M_get() = std::move(*__u);
            } else {
                this->_M_construct(std::move(*__u));
            }
        } else {
            this->_M_reset();
        }

return *this;
    }

template<typename... _Args>
    constexpr
    enable_if_t<is_constructible_v<_Tp, _Args&&...>, _Tp&>
    emplace(_Args&&... __args)
    {
        this->_M_reset();
        this->_M_construct(std::forward<_Args>(__args)...);
        return this->_M_get();
    }

template<typename _Up, typename... _Args>
    constexpr
    enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&,
                                   _Args&&...>, _Tp&>
    emplace(initializer_list<_Up> __il, _Args&&... __args)
    {
        this->_M_reset();
        this->_M_construct(__il, std::forward<_Args>(__args)...);
        return this->_M_get();
    }

// Destructor is implicit, implemented in _Optional_base.

// Swap.
    constexpr void
    swap(optional& __other)
    noexcept(is_nothrow_move_constructible_v<_Tp>
             && is_nothrow_swappable_v<_Tp>)
    {
        using std::swap;

if (this->_M_is_engaged() && __other._M_is_engaged()) {
            swap(this->_M_get(), __other._M_get());
        } else if (this->_M_is_engaged()) {
            __other._M_construct(std::move(this->_M_get()));
            this->_M_destruct();
        } else if (__other._M_is_engaged()) {
            this->_M_construct(std::move(__other._M_get()));
            __other._M_destruct();
        }
    }

// Observers.
    constexpr const _Tp*
    operator->() const
    {
        return std::__addressof(this->_M_get());
    }

constexpr _Tp*
    operator->()
    {
        return std::__addressof(this->_M_get());
    }

constexpr const _Tp&
    operator*() const&
    {
        return this->_M_get();
    }

constexpr _Tp&
    operator*()&
    { return this->_M_get(); }

constexpr _Tp&&
    operator*()&&
    { return std::move(this->_M_get()); }

constexpr const _Tp&&
    operator*() const&&
    {
        return std::move(this->_M_get());
    }

constexpr explicit operator bool() const noexcept
    {
        return this->_M_is_engaged();
    }

constexpr bool has_value() const noexcept
    {
        return this->_M_is_engaged();
    }

constexpr const _Tp&
    value() const&
    {
        return this->_M_is_engaged()
               ? this->_M_get()
               : (__throw_bad_optional_access(), this->_M_get());
    }

constexpr _Tp&
    value()& {
        return this->_M_is_engaged()
        ? this->_M_get()
        : (__throw_bad_optional_access(), this->_M_get());
    }

constexpr _Tp&&
    value()&& {
        return this->_M_is_engaged()
        ? std::move(this->_M_get())
        : (__throw_bad_optional_access(), std::move(this->_M_get()));
    }

constexpr const _Tp&&
    value() const&&
    {
        return this->_M_is_engaged()
               ? std::move(this->_M_get())
               : (__throw_bad_optional_access(), std::move(this->_M_get()));
    }

template<typename _Up>
    constexpr _Tp
    value_or(_Up&& __u) const&
    {
        static_assert(is_copy_constructible_v<_Tp>);
        static_assert(is_convertible_v<_Up&&, _Tp>);

return this->_M_is_engaged()
               ? this->_M_get() : static_cast<_Tp>(std::forward<_Up>(__u));
    }

template<typename _Up>
    constexpr _Tp
    value_or(_Up&& __u) && {
        static_assert(is_move_constructible_v<_Tp>);
        static_assert(is_convertible_v<_Up&&, _Tp>);

return this->_M_is_engaged()
        ? std::move(this->_M_get())
        : static_cast<_Tp>(std::forward<_Up>(__u));
    }

constexpr void reset() noexcept
    {
        this->_M_reset();
    }
};

template<typename _Tp>
using __optional_relop_t =
enable_if_t<is_convertible<_Tp, bool>::value, bool>;

// Comparisons between optional values.
template<typename _Tp, typename _Up>
constexpr auto
operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() == declval<_Up>())> {
    return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
    && (!__lhs || *__lhs == *__rhs);
}

template<typename _Tp, typename _Up>
constexpr auto
operator!=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() != declval<_Up>())> {
    return static_cast<bool>(__lhs) != static_cast<bool>(__rhs)
    || (static_cast<bool>(__lhs) && *__lhs != *__rhs);
}

template<typename _Tp, typename _Up>
constexpr auto
operator<(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() < declval<_Up>())> {
    return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
}

template<typename _Tp, typename _Up>
constexpr auto
operator>(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() > declval<_Up>())> {
    return static_cast<bool>(__lhs) && (!__rhs || *__lhs > *__rhs);
}

template<typename _Tp, typename _Up>
constexpr auto
operator<=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() <= declval<_Up>())> {
    return !__lhs || (static_cast<bool>(__rhs) && *__lhs <= *__rhs);
}

template<typename _Tp, typename _Up>
constexpr auto
operator>=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() >= declval<_Up>())> {
    return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
}

#ifdef __cpp_lib_three_way_comparison
template<typename _Tp, three_way_comparable_with<_Tp> _Up>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y) {
    return __x && __y ? *__x <=> *__y : bool(__x) <=> bool(__y);
}
#endif

// Comparisons with nullopt.
template<typename _Tp>
constexpr bool
operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept {
    return !__lhs;
}

#ifdef __cpp_lib_three_way_comparison
template<typename _Tp>
constexpr strong_ordering
operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept {
    return bool(__x) <=> false;
}
#else
template<typename _Tp>
constexpr bool
operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept {
    return !__rhs;
}

template<typename _Tp>
constexpr bool
operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept {
    return static_cast<bool>(__lhs);
}

template<typename _Tp>
constexpr bool
operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept {
    return static_cast<bool>(__rhs);
}

template<typename _Tp>
constexpr bool
operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept {
    return false;
}

template<typename _Tp>
constexpr bool
operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept {
    return static_cast<bool>(__rhs);
}

template<typename _Tp>
constexpr bool
operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept {
    return static_cast<bool>(__lhs);
}

template<typename _Tp>
constexpr bool
operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept {
    return false;
}

template<typename _Tp>
constexpr bool
operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept {
    return !__lhs;
}

template<typename _Tp>
constexpr bool
operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept {
    return true;
}

template<typename _Tp>
constexpr bool
operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept {
    return true;
}

template<typename _Tp>
constexpr bool
operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept {
    return !__rhs;
}
#endif // three-way-comparison

// Comparisons with value type.
template<typename _Tp, typename _Up>
constexpr auto
operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() == declval<_Up>())>
{ return __lhs && *__lhs == __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
-> __optional_relop_t<decltype(declval<_Up>() == declval<_Tp>())>
{ return __rhs && __lhs == *__rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() != declval<_Up>())>
{ return !__lhs || *__lhs != __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
-> __optional_relop_t<decltype(declval<_Up>() != declval<_Tp>())>
{ return !__rhs || __lhs != *__rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
         -> __optional_relop_t<decltype(declval<_Tp>() < declval<_Up>())>
{ return !__lhs || *__lhs < __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
         -> __optional_relop_t<decltype(declval<_Up>() < declval<_Tp>())>
{ return __rhs && __lhs < *__rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() > declval<_Up>())>
{ return __lhs && *__lhs > __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
-> __optional_relop_t<decltype(declval<_Up>() > declval<_Tp>())>
{ return !__rhs || __lhs > *__rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
         -> __optional_relop_t<decltype(declval<_Tp>() <= declval<_Up>())>
{ return !__lhs || *__lhs <= __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
         -> __optional_relop_t<decltype(declval<_Up>() <= declval<_Tp>())>
{ return __rhs && __lhs <= *__rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
-> __optional_relop_t<decltype(declval<_Tp>() >= declval<_Up>())>
{ return __lhs && *__lhs >= __rhs; }

template<typename _Tp, typename _Up>
constexpr auto
operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
-> __optional_relop_t<decltype(declval<_Up>() >= declval<_Tp>())>
{ return !__rhs || __lhs >= *__rhs; }

#ifdef __cpp_lib_three_way_comparison
template<typename _Tp, typename _Up>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=>(const optional<_Tp>& __x, const _Up& __v) {
    return bool(__x) ? *__x <=> __v : strong_ordering::less;
}
#endif

// Swap and creation functions.

// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2748. swappable traits for optionals
template<typename _Tp>
constexpr inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>>
swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
noexcept(noexcept(__lhs.swap(__rhs))) {
    __lhs.swap(__rhs);
}

template<typename _Tp>
enable_if_t<!(is_move_constructible_v<_Tp> && is_swappable_v<_Tp>)>
swap(optional<_Tp>&, optional<_Tp>&) = delete;

template<typename _Tp>
constexpr optional<decay_t<_Tp>>
make_optional(_Tp&& __t) {
    return optional<decay_t<_Tp>> { std::forward<_Tp>(__t) };
}

template<typename _Tp, typename ..._Args>
constexpr optional<_Tp>
make_optional(_Args&&... __args) {
    return optional<_Tp> { in_place, std::forward<_Args>(__args)... };
}

template<typename _Tp, typename _Up, typename ..._Args>
constexpr optional<_Tp>
make_optional(initializer_list<_Up> __il, _Args&&... __args) {
    return optional<_Tp> { in_place, __il, std::forward<_Args>(__args)... };
}

// Hash.

template<typename _Tp, typename _Up = remove_const_t<_Tp>,
         bool = __poison_hash<_Up>::__enable_hash_call>
struct __optional_hash_call_base {
    size_t
    operator()(const optional<_Tp>& __t) const
    noexcept(noexcept(hash<_Up> {}(*__t)))
    {
        // We pick an arbitrary hash for disengaged optionals which hopefully
        // usual values of _Tp won't typically hash to.
        constexpr size_t __magic_disengaged_hash = static_cast<size_t>(-3333);
        return __t ? hash<_Up> {}(*__t) : __magic_disengaged_hash;
    }
};

template<typename _Tp, typename _Up>
struct __optional_hash_call_base<_Tp, _Up, false> {};

template<typename _Tp>
struct hash<optional<_Tp>>
: private __poison_hash<remove_const_t<_Tp>>,
public __optional_hash_call_base<_Tp> {
    using result_type [[__deprecated__]] = size_t;
    using argument_type [[__deprecated__]] = optional<_Tp>;
};

template<typename _Tp>
struct __is_fast_hash<hash<optional<_Tp>>> : __is_fast_hash<hash<_Tp>> {
};

/// @}

#if __cpp_deduction_guides >= 201606
template <typename _Tp> optional(_Tp) -> optional<_Tp>;
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++17

#endif // _GLIBCXX_OPTIONAL

constexpr std::optional<int> o = []{
    std::optional<int> opt(3);
    std::optional<long> opt2(opt);
    std::optional<int> opt3;
    opt3.swap(opt);
    swap(opt, opt3);

opt2 = std::nullopt;
    opt2 = opt;
    opt2 = 17;
    opt2.reset();

opt.emplace(42);
    return opt;
}();
static_assert(o == 42);

struct X {
    constexpr X() { };
    constexpr ~X() { };
};

constexpr bool f() {
    std::optional<X> o;
    o = X{};
    o = std::nullopt;
    o.emplace(X{});
    return true;
}
static_assert(f());