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 <utility>
  # include <type_traits>
  # include <initializer_list>
  # include <cassert>
  # include <functional>
  # include <string>
  # include <stdexcept>
  
  # 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&>;
  
  
  // 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);
  }
  
  
  #if defined NDEBUG
  # define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
  #else
  # define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(!#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 swap_ns
  {
  using std::swap;
  
  template <class T>
  void adl_swap(T& t, T& u) noexcept(noexcept(swap(t, u)))
  {
      swap(t, u);
  }
  
  } // namespace swap_ns
  
  } // namespace detail
  
  
  constexpr struct trivial_init_t{} trivial_init{};
  
  
  // 20.5.6, In-place construction
  constexpr struct in_place_t{} in_place{};
  
  
  // 20.5.7, Disengaged state indicator
  struct nullopt_t
  {
      struct init{};
      constexpr explicit nullopt_t(init){}
  };
  constexpr nullopt_t nullopt{nullopt_t::init()};
  
  
  // 20.5.8, class bad_optional_access
  class bad_optional_access : public logic_error {
  public:
      explicit bad_optional_access(const string& what_arg) : logic_error{what_arg} {}
      explicit bad_optional_access(const char* what_arg) : logic_error{what_arg} {}
  };
  
  
  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,                          // if possible
      constexpr_optional_base<typename std::remove_const<T>::type>, // use base with trivial destructor
      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>() {};
  
      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_t{}, 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_t{}, 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=(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(detail_::swap_ns::adl_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 bool has_value() 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("bad optional access"), contained_val());
    }
  
    OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
      return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
    }
  
    OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
      if (!initialized()) throw bad_optional_access("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("bad optional access"), contained_val());
      }
  
      T& value() {
          return initialized() ? contained_val() : (throw bad_optional_access("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
  
      // 20.6.3.6, modifiers
      void reset() noexcept { clear(); }
  };
  
  
  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("bad optional access"), *ref);
      }
  
      explicit constexpr operator bool() const noexcept {
          return ref != nullptr;
      }
  
      constexpr bool has_value() 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));
      }
  
      // x.x.x.x, modifiers
      void reset() noexcept { ref = nullptr; }
  };
  
  
  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
  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{};
      }
  };
  }
  
  # undef TR2_OPTIONAL_REQUIRES
  # undef TR2_OPTIONAL_ASSERTED_EXPRESSION
  
  # endif //___OPTIONAL_HPP___
  