optional.hpp

// 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>
# include <iostream>
# include <vector>

# 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 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(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("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

    };


        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;
            }

            friend ostream& operator<<(ostream& os, const optional<T>& o) {
                if (!o) {
                    return os << "None";
                } else {
                    return os << "Some(" << o.value() << ")";
                }
            }

            // 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;
            }

            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
        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 ledger {
    namespace core {
        template <typename T>
        using optional = std::experimental::optional<T>;
    }
}

# undef TR2_OPTIONAL_REQUIRES
# undef TR2_OPTIONAL_ASSERTED_EXPRESSION

# endif //___OPTIONAL_HPP___