// <optional> -*- C++ -*-

// Copyright (C) 2013-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file experimental/optional
 *  This is a TS C++ Library header.
 */

#ifndef _GLIBCXX_EXPERIMENTAL_OPTIONAL
#define _GLIBCXX_EXPERIMENTAL_OPTIONAL 1

/**
 * @defgroup experimental Experimental
 *
 * Components specified by various Technical Specifications.
 *
 * As indicated by the std::experimental namespace and the  header paths,
 * the contents of these Technical Specifications are experimental and not
 * part of the C++ standard. As such the interfaces and implementations may
 * change in the future, and there is <STRONG> no guarantee of compatibility
 * between different GCC releases </STRONG> for these features.
 */

#if __cplusplus >= 201402L

#include <utility>
#include <type_traits>
#include <stdexcept>
#include <new>
#include <initializer_list>
#include <bits/functexcept.h>
#include <bits/functional_hash.h>
#include <bits/enable_special_members.h>
#include <experimental/bits/lfts_config.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace experimental
{
inline namespace fundamentals_v1
{
  /**
   * @defgroup optional Optional values
   * @ingroup experimental
   *
   * Class template for optional values and surrounding facilities, as
   * described in n3793 "A proposal to add a utility class to represent
   * optional objects (Revision 5)".
   *
   * @{
   */

#define __cpp_lib_experimental_optional 201411

  // All subsequent [X.Y.n] references are against n3793.

  // [X.Y.4]
  template<typename _Tp>
    class optional;

  // [X.Y.5]
  /// Tag type for in-place construction.
  struct in_place_t { };

  /// Tag for in-place construction.
  constexpr in_place_t in_place { };

  // [X.Y.6]
  /// 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) { }
  };

  // [X.Y.6]
  /// Tag to disengage optional objects.
  constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };

  // [X.Y.7]
  /**
   *  @brief Exception class thrown when a disengaged optional object is
   *  dereferenced.
   *  @ingroup exceptions
   */
  class bad_optional_access : public logic_error
  {
  public:
    bad_optional_access() : logic_error("bad optional access") { }

    // XXX This constructor is non-standard. Should not be inline
    explicit bad_optional_access(const char* __arg) : logic_error(__arg) { }

    virtual ~bad_optional_access() noexcept = default;
  };

  void
  __throw_bad_optional_access(const char*)
  __attribute__((__noreturn__));

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

#ifndef __cpp_lib_addressof_constexpr
  template<typename _Tp, typename = void>
    struct _Has_addressof_mem : std::false_type { };

  template<typename _Tp>
    struct _Has_addressof_mem<_Tp,
         __void_t<decltype( std::declval<const _Tp&>().operator&() )>
      >
    : std::true_type { };

  template<typename _Tp, typename = void>
    struct _Has_addressof_free : std::false_type { };

  template<typename _Tp>
    struct _Has_addressof_free<_Tp,
         __void_t<decltype( operator&(std::declval<const _Tp&>()) )>
      >
    : std::true_type { };

  /**
    * @brief Trait that detects the presence of an overloaded unary operator&.
    *
    * Practically speaking this detects the presence of such an operator when
    * called on a const-qualified lvalue (e.g.
    * declval<const _Tp&>().operator&()).
    */
  template<typename _Tp>
    struct _Has_addressof
    : std::__or_<_Has_addressof_mem<_Tp>, _Has_addressof_free<_Tp>>::type
    { };

  /**
    * @brief An overload that attempts to take the address of an lvalue as a
    * constant expression. Falls back to __addressof in the presence of an
    * overloaded addressof operator (unary operator&), in which case the call
    * will not be a constant expression.
    */
  template<typename _Tp>
    constexpr
    enable_if_t<!_Has_addressof<_Tp>::value, _Tp*>
    __constexpr_addressof(_Tp& __t)
    { return &__t; }

  /**
    * @brief Fallback overload that defers to __addressof.
    */
  template<typename _Tp>
    inline
    enable_if_t<_Has_addressof<_Tp>::value, _Tp*>
    __constexpr_addressof(_Tp& __t)
    { return std::__addressof(__t); }
#endif // __cpp_lib_addressof_constexpr

  /**
    * @brief Class template that holds the necessary state for @ref optional
    * and that has the responsibility for construction and the special members.
    *
    * Such a separate base class template is necessary in order to
    * conditionally enable the special members (e.g. copy/move constructors).
    * Note that this means that @ref _Optional_base implements the
    * functionality for copy and move assignment, but not for converting
    * assignment.
    *
    * @see optional, _Enable_special_members
    */
  template<typename _Tp, bool _ShouldProvideDestructor =
	   !is_trivially_destructible<_Tp>::value>
    class _Optional_base
    {
    private:
      // Remove const to avoid prohibition of reusing object storage for
      // const-qualified types in [3.8/9]. This is strictly internal
      // and even optional itself is oblivious to it.
      using _Stored_type = remove_const_t<_Tp>;

    public:
      // [X.Y.4.1] Constructors.

      // Constructors for disengaged optionals.
      constexpr _Optional_base() noexcept
      : _M_empty{} { }

      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }

      // Constructors for engaged optionals.
      template<typename... _Args>
        constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
        : _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }

      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible<_Tp,
                                            initializer_list<_Up>&,
                                            _Args&&...>::value,
                           int>...>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true) { }

      // Copy and move constructors.
      _Optional_base(const _Optional_base& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(__other._M_get());
      }

      _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible<_Tp>())
      {
        if (__other._M_engaged)
          this->_M_construct(std::move(__other._M_get()));
      }

      // [X.Y.4.3] (partly) Assignment.
      _Optional_base&
      operator=(const _Optional_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();
	  }

        return *this;
      }

      _Optional_base&
      operator=(_Optional_base&& __other)
      noexcept(__and_<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();
	  }
	return *this;
      }

      // [X.Y.4.2] Destructor.
      ~_Optional_base()
      {
        if (this->_M_engaged)
          this->_M_payload.~_Stored_type();
      }

      // The following functionality is also needed by optional, hence the
      // protected accessibility.
    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this->_M_engaged; }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&
      _M_get() noexcept
      { return _M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return _M_payload; }

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

      void
      _M_destruct()
      {
        this->_M_engaged = false;
        this->_M_payload.~_Stored_type();
      }

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

    private:
      struct _Empty_byte { };
      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };

  /// Partial specialization that is exactly identical to the primary template
  /// save for not providing a destructor, to fulfill triviality requirements.
  template<typename _Tp>
    class _Optional_base<_Tp, false>
    {
    private:
      using _Stored_type = remove_const_t<_Tp>;

    public:
      constexpr _Optional_base() noexcept
      : _M_empty{} { }

      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }

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

      template<typename _Up, typename... _Args,
               enable_if_t<is_constructible<_Tp,
                                            initializer_list<_Up>&,
                                            _Args&&...>::value,
			   int>...>
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list<_Up> __il,
                                          _Args&&... __args)
        : _M_payload(__il, std::forward<_Args>(__args)...),
          _M_engaged(true) { }

      _Optional_base(const _Optional_base& __other)
      {
        if (__other._M_engaged)
          this->_M_construct(__other._M_get());
      }

      _Optional_base(_Optional_base&& __other)
      noexcept(is_nothrow_move_constructible<_Tp>())
      {
        if (__other._M_engaged)
          this->_M_construct(std::move(__other._M_get()));
      }

      _Optional_base&
      operator=(const _Optional_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();
	  }
	return *this;
      }

      _Optional_base&
      operator=(_Optional_base&& __other)
      noexcept(__and_<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();
	  }
	return *this;
      }

      // Sole difference
      // ~_Optional_base() noexcept = default;

    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this->_M_engaged; }

      _Tp&
      _M_get() noexcept
      { return _M_payload; }

      constexpr const _Tp&
      _M_get() const noexcept
      { return _M_payload; }

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

      void
      _M_destruct()
      {
        this->_M_engaged = false;
        this->_M_payload.~_Stored_type();
      }

      void
      _M_reset()
      {
        if (this->_M_engaged)
          this->_M_destruct();
      }

    private:
      struct _Empty_byte { };
      union
      {
	_Empty_byte _M_empty;
	_Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };

  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<_Tp>::value,
        // Copy assignment.
        __and_<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>::value,
        // Move constructor.
        is_move_constructible<_Tp>::value,
        // Move assignment.
        __and_<is_move_constructible<_Tp>, is_move_assignable<_Tp>>::value,
        // Unique tag type.
        optional<_Tp>>
    {
      static_assert(__and_<__not_<is_same<remove_cv_t<_Tp>, nullopt_t>>,
			   __not_<is_same<remove_cv_t<_Tp>, in_place_t>>,
			   __not_<is_reference<_Tp>>>(),
                    "Invalid instantiation of optional<T>");

    private:
      using _Base = _Optional_base<_Tp>;

    public:
      using value_type = _Tp;

      // _Optional_base has the responsibility for construction.
      using _Base::_Base;

      constexpr optional() = default;
      // Conv                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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