text: // Copyright (C) 2011 - 2012 Andrzej Krzemienski.
  //
  // Use, modification, and distribution is subject to the Boost Software
  // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
  // http://www.boost.org/LICENSE_1_0.txt)
  //
  // The idea and interface is based on Boost.Optional library
  // authored by Fernando Luis Cacciola Carballal
  
  #ifndef ___OPTIONAL_HPP___
  #define ___OPTIONAL_HPP___
  
  #include <experimental/fundamental/v3/in_place.hpp>
  #include <experimental/fundamental/v3/optional/bad_optional_access.hpp>
  #include <experimental/fundamental/v3/optional/nullopt.hpp>
  #include <experimental/fundamental/v3/utility/constexpr_utility.hpp>
  #include <experimental/nullable.hpp>
  #include <cassert>
  #include <functional>
  #include <initializer_list>
  #include <stdexcept>
  #include <string>
  #include <type_traits>
  #include <utility>
  #include <utility>
  
  #define TR2_OPTIONAL_REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false
  
  #if defined __GNUC__ // NOTE: GNUC is also defined for Clang
  #if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
  #define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
  #elif (__GNUC__ > 4)
  #define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
  #endif
  #
  #if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)
  #define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
  #elif (__GNUC__ > 4)
  #define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
  #endif
  #
  #if (__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1)
  #define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
  #elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)
  #define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
  #elif (__GNUC__ > 4)
  #define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
  #endif
  #endif
  #
  #if defined __clang_major__
  #if (__clang_major__ == 3 && __clang_minor__ >= 5)
  #define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
  #elif (__clang_major__ > 3)
  #define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
  #endif
  #if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
  #define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
  #elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
  #define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
  #endif
  #endif
  #
  #if defined _MSC_VER
  #if (_MSC_VER >= 1900)
  #define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
  #endif
  #endif
  
  #if defined __clang__
  #if (__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9)
  #define OPTIONAL_HAS_THIS_RVALUE_REFS 1
  #else
  #define OPTIONAL_HAS_THIS_RVALUE_REFS 0
  #endif
  #elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
  #define OPTIONAL_HAS_THIS_RVALUE_REFS 1
  #elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
  #define OPTIONAL_HAS_THIS_RVALUE_REFS 1
  #else
  #define OPTIONAL_HAS_THIS_RVALUE_REFS 0
  #endif
  
  #if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
  #define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1
  #define OPTIONAL_CONSTEXPR_INIT_LIST constexpr
  #else
  #define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0
  #define OPTIONAL_CONSTEXPR_INIT_LIST
  #endif
  
  #if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)
  #define OPTIONAL_HAS_MOVE_ACCESSORS 1
  #else
  #define OPTIONAL_HAS_MOVE_ACCESSORS 0
  #endif
  
  #// In C++11 constexpr implies const, so we need to make non-const members also non-constexpr
  #if (defined __cplusplus) && (__cplusplus == 201103L)
  #define OPTIONAL_MUTABLE_CONSTEXPR
  #else
  #define OPTIONAL_MUTABLE_CONSTEXPR constexpr
  #endif
  
  namespace std
  {
  
  namespace experimental
  {
  
  // BEGIN workaround for missing is_trivially_destructible
  #if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
  // leave it: it is already there
  #elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
  // leave it: it is already there
  #elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
  // leave it: it is already there
  #elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
  // leave it: the user doesn't want it
  #else
  template <typename T>
  using is_trivially_destructible = std::has_trivial_destructor<T>;
  #endif
  // END workaround for missing is_trivially_destructible
  
  #if (defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___)
  // leave it; our metafunctions are already defined.
  #elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
  // leave it; our metafunctions are already defined.
  #elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
  // leave it: it is already there
  #elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
  // leave it: the user doesn't want it
  #else
  
  // workaround for missing traits in GCC and CLANG
  template <class T>
  struct is_nothrow_move_constructible
  {
  	constexpr static bool value = std::is_nothrow_constructible<T, T &&>::value;
  };
  
  template <class T, class U>
  struct is_assignable
  {
  	template <class X, class Y>
  	constexpr static bool has_assign(...)
  	{
  		return false;
  	}
  
  	template <class X, class Y, size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true))>
  	// the comma operator is necessary for the cases where operator= returns void
  	constexpr static bool has_assign(bool)
  	{
  		return true;
  	}
  
  	constexpr static bool value = has_assign<T, U>(true);
  };
  
  template <class T>
  struct is_nothrow_move_assignable
  {
  	template <class X, bool has_any_move_assign>
  	struct has_nothrow_move_assign
  	{
  		constexpr static bool value = false;
  	};
  
  	template <class X>
  	struct has_nothrow_move_assign<X, true>
  	{
  		constexpr static bool value = noexcept(std::declval<X &>() = std::declval<X &&>());
  	};
  
  	constexpr static bool value = has_nothrow_move_assign<T, is_assignable<T &, T &&>::value>::value;
  };
  // end workaround
  
  #endif
  
  // 20.5.4, optional for object types
  template <class T>
  class optional;
  
  // 20.5.5, optional for lvalue reference types
  template <class T>
  class optional<T &>;
  
  #if 0
  
  // workaround: std utility functions aren't constexpr yet
  template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type& t) noexcept
  {
    return static_cast<T&&>(t);
  }
  
  template <class T> inline constexpr T&& constexpr_forward(typename std::remove_reference<T>::type&& t) noexcept
  {
      static_assert(!std::is_lvalue_reference<T>::value, "!!");
      return static_cast<T&&>(t);
  }
  
  template <class T> inline constexpr typename std::remove_reference<T>::type&& constexpr_move(T&& t) noexcept
  {
      return static_cast<typename std::remove_reference<T>::type&&>(t);
  }
  
  #endif
  
  #if defined NDEBUG
  #define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
  #else
  #define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([] { assert(false && 0 != #CHECK); }(), (EXPR)))
  #endif
  
  namespace detail_
  {
  
  // static_addressof: a constexpr version of addressof
  template <typename T>
  struct has_overloaded_addressof
  {
  	template <class X>
  	constexpr static bool has_overload(...)
  	{
  		return false;
  	}
  
  	template <class X, size_t S = sizeof(std::declval<X &>().operator&())>
  	constexpr static bool has_overload(bool)
  	{
  		return true;
  	}
  
  	constexpr static bool value = has_overload<T>(true);
  };
  
  template <typename T, TR2_OPTIONAL_REQUIRES(!has_overloaded_addressof<T>)>
  constexpr T *static_addressof(T &ref)
  {
  	return &ref;
  }
  
  template <typename T, TR2_OPTIONAL_REQUIRES(has_overloaded_addressof<T>)>
  T *static_addressof(T &ref)
  {
  	return std::addressof(ref);
  }
  
  // the call to convert<A>(b) has return type A and converts b to type A iff b decltype(b) is implicitly convertible to A
  template <class U>
  constexpr U convert(U v)
  {
  	return v;
  }
  
  } // namespace detail_
  
  constexpr struct trivial_init_t
  {
  } trivial_init{};
  
  template <class T>
  union storage_t
  {
  	unsigned char dummy_;
  	T             value_;
  
  	constexpr storage_t(trivial_init_t) noexcept : dummy_() {}
  
  	template <class... Args>
  	constexpr storage_t(Args &&... args) : value_(constexpr_forward<Args>(args)...)
  	{
  	}
  
  	~storage_t() {}
  };
  
  template <class T>
  union constexpr_storage_t
  {
  	unsigned char dummy_;
  	T             value_;
  
  	constexpr constexpr_storage_t(trivial_init_t) noexcept : dummy_() {}
  
  	template <class... Args>
  	constexpr constexpr_storage_t(Args &&... args) : value_(constexpr_forward<Args>(args)...)
  	{
  	}
  
  	~constexpr_storage_t() = default;
  };
  
  template <class T>
  struct optional_base
  {
  	bool         init_;
  	storage_t<T> storage_;
  
  	constexpr optional_base() noexcept : init_(false), storage_(trivial_init) {}
  
  	explicit constexpr optional_base(const T &v) : init_(true), storage_(v) {}
  
  	explicit constexpr optional_base(T &&v) : init_(true), storage_(constexpr_move(v)) {}
  
  	template <class... Args>
  	explicit optional_base(in_place_t, Args &&... args)
  	        : init_(true), storage_(constexpr_forward<Args>(args)...)
  	{
  	}
  
  	template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
  	explicit optional_base(in_place_t, std::initializer_list<U> il, Args &&... args)
  	        : init_(true), storage_(il, std::forward<Args>(args)...)
  	{
  	}
  
  	~optional_base()
  	{
  		if (init_)
  			storage_.value_.T::~T();
  	}
  };
  
  template <class T>
  struct constexpr_optional_base
  {
  	bool                   init_;
  	constexpr_storage_t<T> storage_;
  
  	constexpr constexpr_optional_base() noexcept : init_(false), storage_(trivial_init) {}
  
  	explicit constexpr constexpr_optional_base(const T &v) : init_(true), storage_(v) {}
  
  	explicit constexpr constexpr_optional_base(T &&v) : init_(true), storage_(constexpr_move(v)) {}
  
  	template <class... Args>
  	explicit constexpr constexpr_optional_base(in_place_t, Args &&... args)
  	        : init_(true), storage_(constexpr_forward<Args>(args)...)
  	{
  	}
  
  	template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
  	OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, std::initializer_list<U> il, Args &&... args)
  	        : init_(true), storage_(il, std::forward<Args>(args)...)
  	{
  	}
  
  	~constexpr_optional_base() = default;
  };
  
  template <class T>
  using OptionalBase = typename std::conditional<
          is_trivially_destructible<T>::value,
          constexpr_optional_base<typename std::remove_const<T>::type>,
          optional_base<typename std::remove_const<T>::type>>::type;
  
  template <class T>
  class optional : private OptionalBase<T> {
  	static_assert(!std::is_same<typename std::decay<T>::type, nullopt_t>::value, "bad T");
  	static_assert(!std::is_same<typename std::decay<T>::type, in_place_t>::value, "bad T");
  
  	constexpr bool                       initialized() const noexcept { return OptionalBase<T>::init_; }
  	typename std::remove_const<T>::type *dataptr() { return std::addressof(OptionalBase<T>::storage_.value_); }
  	constexpr const T *                  dataptr() const { return detail_::static_addressof(OptionalBase<T>::storage_.value_); }
  
  #if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
  	constexpr const T &contained_val() const &
  	{
  		return OptionalBase<T>::storage_.value_;
  	}
  #if OPTIONAL_HAS_MOVE_ACCESSORS == 1
  	OPTIONAL_MUTABLE_CONSTEXPR T &&contained_val() &&
  	{
  		return std::move(OptionalBase<T>::storage_.value_);
  	}
  	OPTIONAL_MUTABLE_CONSTEXPR T &contained_val() & { return OptionalBase<T>::storage_.value_; }
  #else
  	T &contained_val() &
  	{
  		return OptionalBase<T>::storage_.value_;
  	}
  	T &&contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
  #endif
  #else
  	constexpr const T &contained_val() const
  	{
  		return OptionalBase<T>::storage_.value_;
  	}
  	T &contained_val() { return OptionalBase<T>::storage_.value_; }
  #endif
  
  	void clear() noexcept
  	{
  		if (initialized())
  			dataptr()->T::~T();
  		OptionalBase<T>::init_ = false;
  	}
  
  	template <class... Args>
  	void initialize(Args &&... args) noexcept(noexcept(T(std::forward<Args>(args)...)))
  	{
  		assert(!OptionalBase<T>::init_);
  		::new (static_cast<void *>(dataptr())) T(std::forward<Args>(args)...);
  		OptionalBase<T>::init_ = true;
  	}
  
  	template <class U, class... Args>
  	void initialize(std::initializer_list<U> il, Args &&... args) noexcept(noexcept(T(il, std::forward<Args>(args)...)))
  	{
  		assert(!OptionalBase<T>::init_);
  		::new (static_cast<void *>(dataptr())) T(il, std::forward<Args>(args)...);
  		OptionalBase<T>::init_ = true;
  	}
  
  public:
  	typedef T value_type;
  
  	// 20.5.5.1, constructors
  	constexpr optional() noexcept : OptionalBase<T>() {}
  	constexpr optional(nullopt_t) noexcept : OptionalBase<T>() {}
  	constexpr optional(none_t) noexcept : OptionalBase<T>() {}
  
  	optional(const optional &rhs)
  	        : OptionalBase<T>()
  	{
  		if (rhs.initialized())
  		{
  			::new (static_cast<void *>(dataptr())) T(*rhs);
  			OptionalBase<T>::init_ = true;
  		}
  	}
  
  	optional(optional &&rhs) noexcept(is_nothrow_move_constructible<T>::value)
  	        : OptionalBase<T>()
  	{
  		if (rhs.initialized())
  		{
  			::new (static_cast<void *>(dataptr())) T(std::move(*rhs));
  			OptionalBase<T>::init_ = true;
  		}
  	}
  
  	constexpr optional(const T &v) : OptionalBase<T>(v) {}
  
  	constexpr optional(T &&v) : OptionalBase<T>(constexpr_move(v)) {}
  
  	template <class... Args>
  	explicit constexpr optional(in_place_t, Args &&... args)
  	        : OptionalBase<T>(in_place, constexpr_forward<Args>(args)...)
  	{
  	}
  
  	template <class U, class... Args, TR2_OPTIONAL_REQUIRES(is_constructible<T, std::initializer_list<U>>)>
  	OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, std::initializer_list<U> il, Args &&... args)
  	        : OptionalBase<T>(in_place, il, constexpr_forward<Args>(args)...)
  	{
  	}
  
  	// 20.5.4.2, Destructor
  	~optional() = default;
  
  	// 20.5.4.3, assignment
  	optional &operator=(nullopt_t) noexcept
  	{
  		clear();
  		return *this;
  	}
  
  	optional &operator=(none_t) noexcept
  	{
  		clear();
  		return *this;
  	}
  
  	void reset() noexcept
  	{
  		this->clear();
  	}
  	optional &operator=(const optional &rhs)
  	{
  		if (initialized() == true && rhs.initialized() == false)
  			clear();
  		else if (initialized() == false && rhs.initialized() == true)
  			initialize(*rhs);
  		else if (initialized() == true && rhs.initialized() == true)
  			contained_val() = *rhs;
  		return *this;
  	}
  
  	optional &operator=(optional &&rhs) noexcept(is_nothrow_move_assignable<T>::value &&is_nothrow_move_constructible<T>::value)
  	{
  		if (initialized() == true && rhs.initialized() == false)
  			clear();
  		else if (initialized() == false && rhs.initialized() == true)
  			initialize(std::move(*rhs));
  		else if (initialized() == true && rhs.initialized() == true)
  			contained_val() = std::move(*rhs);
  		return *this;
  	}
  
  	template <class U>
  	auto operator=(U &&v)
  	        -> typename enable_if<
  	                is_same<typename decay<U>::type, T>::value,
  	                optional &>::type
  	{
  		if (initialized())
  		{
  			contained_val() = std::forward<U>(v);
  		}
  		else
  		{
  			initialize(std::forward<U>(v));
  		}
  		return *this;
  	}
  
  	template <class... Args>
  	void emplace(Args &&... args)
  	{
  		clear();
  		initialize(std::forward<Args>(args)...);
  	}
  
  	template <class U, class... Args>
  	void emplace(initializer_list<U> il, Args &&... args)
  	{
  		clear();
  		initialize<U, Args...>(il, std::forward<Args>(args)...);
  	}
  
  	// 20.5.4.4, Swap
  	void swap(optional<T> &rhs) noexcept(is_nothrow_move_constructible<T>::value &&noexcept(swap(declval<T &>(), declval<T &>())))
  	{
  		if (initialized() == true && rhs.initialized() == false)
  		{
  			rhs.initialize(std::move(**this));
  			clear();
  		}
  		else if (initialized() == false && rhs.initialized() == true)
  		{
  			initialize(std::move(*rhs));
  			rhs.clear();
  		}
  		else if (initialized() == true && rhs.initialized() == true)
  		{
  			using std::swap;
  			swap(**this, *rhs);
  		}
  	}
  
  	// 20.5.4.5, Observers
  
  	explicit constexpr operator bool() const noexcept { return initialized(); }
  
  	constexpr T const *operator->() const
  	{
  		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
  	}
  
  #if OPTIONAL_HAS_MOVE_ACCESSORS == 1
  
  	OPTIONAL_MUTABLE_CONSTEXPR T *operator->()
  	{
  		assert(initialized());
  		return dataptr();
  	}
  
  	constexpr T const &operator*() const &
  	{
  		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
  	}
  
  	OPTIONAL_MUTABLE_CONSTEXPR T &operator*() &
  	{
  		assert(initialized());
  		return contained_val();
  	}
  
  	OPTIONAL_MUTABLE_CONSTEXPR T &&operator*() &&
  	{
  		assert(initialized());
  		return constexpr_move(contained_val());
  	}
  
  	constexpr T const &value() const &
  	{
  		return initialized() ? contained_val() : (throw bad_optional_access(), contained_val());
  	}
  
  	OPTIONAL_MUTABLE_CONSTEXPR T &value() &
  	{
  		return initialized() ? contained_val() : (throw bad_optional_access(), contained_val());
  	}
  
  	OPTIONAL_MUTABLE_CONSTEXPR T &&value() &&
  	{
  		if (!initialized())
  			throw bad_optional_access();
  		return std::move(contained_val());
  	}
  
  #else
  
  	T *operator->()
  	{
  		assert(initialized());
  		return dataptr();
  	}
  
  	constexpr T const &operator*() const
  	{
  		return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
  	}
  
  	T &operator*()
  	{
  		assert(initialized());
  		return contained_val();
  	}
  
  	constexpr T const &value() const
  	{
  		return initialized() ? contained_val() : (throw bad_optional_access(), contained_val());
  	}
  
  	T &value()
  	{
  		return initialized() ? contained_val() : (throw bad_optional_access(), contained_val());
  	}
  
  #endif
  
  #if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
  
  	template <class V>
  	constexpr T value_or(V &&v) const &
  	{
  		return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
  	}
  
  #if OPTIONAL_HAS_MOVE_ACCESSORS == 1
  
  	template <class V>
  	OPTIONAL_MUTABLE_CONSTEXPR T value_or(V &&v) &&
  	{
  		return *this ? constexpr_move(const_cast<optional<T> &>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
  	}
  
  #else
  
  	template <class V>
  	T value_or(V &&v) &&
  	{
  		return *this ? constexpr_move(const_cast<optional<T> &>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
  	}
  
  #endif
  
  #else
  
  	template <class V>
  	constexpr T value_or(V &&v) const
  	{
  		return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
  	}
  
  #endif
  };
  
  template <class T>
  class optional<T &> {
  	static_assert(!std::is_same<T, nullopt_t>::value, "bad T");
  	static_assert(!std::is_same<T, in_place_t>::value, "bad T");
  	T *ref;
  
  public:
  	// 20.5.5.1, construction/destruction
  	constexpr optional() noexcept : ref(nullptr) {}
  
  	constexpr optional(nullopt_t) noexcept : ref(nullptr) {}
  
  	constexpr optional(T &v) noexcept : ref(detail_::static_addressof(v)) {}
  
  	optional(T &&) = delete;
  
  	constexpr optional(const optional &rhs) noexcept : ref(rhs.ref) {}
  
  	explicit constexpr optional(in_place_t, T &v) noexcept : ref(detail_::static_addressof(v)) {}
  
  	explicit optional(in_place_t, T &&) = delete;
  
  	~optional() = default;
  
  	// 20.5.5.2, mutation
  	optional &operator=(nullopt_t) noexcept
  	{
  		ref = nullptr;
  		return *this;
  	}
  
  	// optional& operator=(const optional& rhs) noexcept {
  	// ref = rhs.ref;
  	// return *this;
  	// }
  
  	// optional& operator=(optional&& rhs) noexcept {
  	// ref = rhs.ref;
  	// return *this;
  	// }
  
  	template <typename U>
  	auto operator=(U &&rhs) noexcept
  	        -> typename enable_if<
  	                is_same<typename decay<U>::type, optional<T &>>::value,
  	                optional &>::type
  	{
  		ref = rhs.ref;
  		return *this;
  	}
  
  	template <typename U>
  	auto operator=(U &&rhs) noexcept
  	        -> typename enable_if<
  	                !is_same<typename decay<U>::type, optional<T &>>::value,
  	                optional &>::type = delete;
  
  	void emplace(T &v) noexcept
  	{
  		ref = detail_::static_addressof(v);
  	}
  
  	void emplace(T &&) = delete;
  
  	void swap(optional<T &> &rhs) noexcept
  	{
  		std::swap(ref, rhs.ref);
  	}
  
  	// 20.5.5.3, observers
  	constexpr T *operator->() const
  	{
  		return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
  	}
  
  	constexpr T &operator*() const
  	{
  		return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
  	}
  
  	constexpr T &value() const
  	{
  		return ref ? *ref : (throw bad_optional_access(), *ref);
  	}
  
  	explicit constexpr operator bool() const noexcept
  	{
  		return ref != nullptr;
  	}
  
  	template <class V>
  	constexpr typename decay<T>::type value_or(V &&v) const
  	{
  		return *this ? **this : detail_::convert<typename decay<T>::type>(constexpr_forward<V>(v));
  	}
  };
  
  template <class T>
  class optional<T &&> {
  	static_assert(sizeof(T) == 0, "optional rvalue references disallowed");
  };
  
  // 20.5.8, Relational operators
  template <class T>
  constexpr bool operator==(const optional<T> &x, const optional<T> &y)
  {
  	return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y;
  }
  
  template <class T>
  constexpr bool operator!=(const optional<T> &x, const optional<T> &y)
  {
  	return !(x == y);
  }
  
  template <class T>
  constexpr bool operator<(const optional<T> &x, const optional<T> &y)
  {
  	return (!y) ? false : (!x) ? true : *x < *y;
  }
  
  template <class T>
  constexpr bool operator>(const optional<T> &x, const optional<T> &y)
  {
  	return (y < x);
  }
  
  template <class T>
  constexpr bool operator<=(const optional<T> &x, const optional<T> &y)
  {
  	return !(y < x);
  }
  
  template <class T>
  constexpr bool operator>=(const optional<T> &x, const optional<T> &y)
  {
  	return !(x < y);
  }
  
  // 20.5.9, Comparison with nullopt
  constexpr bool operator==(nullopt_t, nullopt_t) noexcept
  {
  	return true;
  }
  constexpr bool operator!=(nullopt_t, nullopt_t) noexcept
  {
  	return false;
  }
  
  template <class T>
  constexpr bool operator==(const optional<T> &x, nullopt_t) noexcept
  {
  	return (!x);
  }
  
  template <class T>
  constexpr bool operator==(nullopt_t, const optional<T> &x) noexcept
  {
  	return (!x);
  }
  
  template <class T>
  constexpr bool operator!=(const optional<T> &x, nullopt_t) noexcept
  {
  	return bool(x);
  }
  
  template <class T>
  constexpr bool operator!=(nullopt_t, const optional<T> &x) noexcept
  {
  	return bool(x);
  }
  
  template <class T>
  constexpr bool operator<(const optional<T> &, nullopt_t) noexcept
  {
  	return false;
  }
  
  template <class T>
  constexpr bool operator<(nullopt_t, const optional<T> &x) noexcept
  {
  	return bool(x);
  }
  
  template <class T>
  constexpr bool operator<=(const optional<T> &x, nullopt_t) noexcept
  {
  	return (!x);
  }
  
  template <class T>
  constexpr bool operator<=(nullopt_t, const optional<T> &) noexcept
  {
  	return true;
  }
  
  template <class T>
  constexpr bool operator>(const optional<T> &x, nullopt_t) noexcept
  {
  	return bool(x);
  }
  
  template <class T>
  constexpr bool operator>(nullopt_t, const optional<T> &) noexcept
  {
  	return false;
  }
  
  template <class T>
  constexpr bool operator>=(const optional<T> &, nullopt_t) noexcept
  {
  	return true;
  }
  
  template <class T>
  constexpr bool operator>=(nullopt_t, const optional<T> &x) noexcept
  {
  	return (!x);
  }
  
  // 20.5.10, Comparison with T
  template <class T>
  constexpr bool operator==(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x == v : false;
  }
  
  template <class T>
  constexpr bool operator==(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v == *x : false;
  }
  
  template <class T>
  constexpr bool operator!=(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x != v : true;
  }
  
  template <class T>
  constexpr bool operator!=(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v != *x : true;
  }
  
  template <class T>
  constexpr bool operator<(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x < v : true;
  }
  
  template <class T>
  constexpr bool operator>(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v > *x : true;
  }
  
  template <class T>
  constexpr bool operator>(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x > v : false;
  }
  
  template <class T>
  constexpr bool operator<(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v < *x : false;
  }
  
  template <class T>
  constexpr bool operator>=(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x >= v : false;
  }
  
  template <class T>
  constexpr bool operator<=(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v <= *x : false;
  }
  
  template <class T>
  constexpr bool operator<=(const optional<T> &x, const T &v)
  {
  	return bool(x) ? *x <= v : true;
  }
  
  template <class T>
  constexpr bool operator>=(const T &v, const optional<T> &x)
  {
  	return bool(x) ? v >= *x : true;
  }
  
  // Comparison of optional<T&> with T
  template <class T>
  constexpr bool operator==(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x == v : false;
  }
  
  template <class T>
  constexpr bool operator==(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v == *x : false;
  }
  
  template <class T>
  constexpr bool operator!=(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x != v : true;
  }
  
  template <class T>
  constexpr bool operator!=(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v != *x : true;
  }
  
  template <class T>
  constexpr bool operator<(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x < v : true;
  }
  
  template <class T>
  constexpr bool operator>(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v > *x : true;
  }
  
  template <class T>
  constexpr bool operator>(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x > v : false;
  }
  
  template <class T>
  constexpr bool operator<(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v < *x : false;
  }
  
  template <class T>
  constexpr bool operator>=(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x >= v : false;
  }
  
  template <class T>
  constexpr bool operator<=(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v <= *x : false;
  }
  
  template <class T>
  constexpr bool operator<=(const optional<T &> &x, const T &v)
  {
  	return bool(x) ? *x <= v : true;
  }
  
  template <class T>
  constexpr bool operator>=(const T &v, const optional<T &> &x)
  {
  	return bool(x) ? v >= *x : true;
  }
  
  // Comparison of optional<T const&> with T
  template <class T>
  constexpr bool operator==(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x == v : false;
  }
  
  template <class T>
  constexpr bool operator==(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v == *x : false;
  }
  
  template <class T>
  constexpr bool operator!=(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x != v : true;
  }
  
  template <class T>
  constexpr bool operator!=(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v != *x : true;
  }
  
  template <class T>
  constexpr bool operator<(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x < v : true;
  }
  
  template <class T>
  constexpr bool operator>(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v > *x : true;
  }
  
  template <class T>
  constexpr bool operator>(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x > v : false;
  }
  
  template <class T>
  constexpr bool operator<(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v < *x : false;
  }
  
  template <class T>
  constexpr bool operator>=(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x >= v : false;
  }
  
  template <class T>
  constexpr bool operator<=(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v <= *x : false;
  }
  
  template <class T>
  constexpr bool operator<=(const optional<const T &> &x, const T &v)
  {
  	return bool(x) ? *x <= v : true;
  }
  
  template <class T>
  constexpr bool operator>=(const T &v, const optional<const T &> &x)
  {
  	return bool(x) ? v >= *x : true;
  }
  
  // 20.5.12, Specialized algorithms
  template <class T>
  void swap(optional<T> &x, optional<T> &y) noexcept(noexcept(x.swap(y)))
  {
  	x.swap(y);
  }
  
  template <class T>
  constexpr optional<typename decay<T>::type> make_optional(T &&v)
  {
  	return optional<typename decay<T>::type>(constexpr_forward<T>(v));
  }
  
  template <class X>
  constexpr optional<X &> make_optional(reference_wrapper<X> v)
  {
  	return optional<X &>(v.get());
  }
  
  } // namespace experimental
  } // namespace std
  
  namespace std
  {
  template <typename T>
  struct hash<std::experimental::optional<T>>
  {
  	typedef typename hash<T>::result_type  result_type;
  	typedef std::experimental::optional<T> argument_type;
  
  	constexpr result_type operator()(argument_type const &arg) const
  	{
  		return arg ? std::hash<T>{}(*arg) : result_type{};
  	}
  };
  
  template <typename T>
  struct hash<std::experimental::optional<T &>>
  {
  	typedef typename hash<T>::result_type    result_type;
  	typedef std::experimental::optional<T &> argument_type;
  
  	constexpr result_type operator()(argument_type const &arg) const
  	{
  		return arg ? std::hash<T>{}(*arg) : result_type{};
  	}
  };
  } // namespace std
  
  #undef TR2_OPTIONAL_REQUIRES
  #undef TR2_OPTIONAL_ASSERTED_EXPRESSION
  
  #endif //___OPTIONAL_HPP___
  