// Copyright (C) 2011 - 2012 Andrzej Krzemienski.
//
// Use, modification, and distribution is subject to the Boost Software
// License, Version 1.0, as follows:
//
// Boost Software License - Version 1.0 - August 17th, 2003
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
//
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//
// 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 REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false

# 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
# else
#  define OPTIONAL_HAS_THIS_RVALUE_REFS 0
# endif


namespace std{

namespace experimental{


// 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);
}

template<class _Ty> inline constexpr _Ty * constexpr_addressof(_Ty& _Val)
{
    return ((_Ty *) &(char&)_Val);
}


#if defined NDEBUG
# define ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
#else
# define ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(#CHECK && false);}(), (EXPR)))
#endif


template <typename T>
struct has_overloaded_addressof
{
  template <class X>
  static constexpr bool has_overload(...) { return false; }

  template <class X, size_t S = sizeof(std::declval< X&>().operator&()) >
  static constexpr bool has_overload(bool) { return true; }

  constexpr static bool value = has_overload<T>(true);
};



template <typename T, REQUIRES(!has_overloaded_addressof<T>)>
constexpr T* static_addressof(T& ref)
{
  return &ref;
}

template <typename T, REQUIRES(has_overloaded_addressof<T>)>
T* static_addressof(T& ref)
{
  return std::addressof(ref);
}



template <class U>
struct is_not_optional
{
  constexpr static bool value = true;
};

template <class T>
struct is_not_optional<optional<T>>
{
  constexpr static bool value = false;
};


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 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;
};


constexpr struct only_set_initialized_t{} only_set_initialized{};


template <class T>
struct optional_base
{
    bool init_;
    storage_t<T> storage_;

    constexpr optional_base() noexcept : init_(false), storage_(trivial_init) {};

    constexpr explicit optional_base(only_set_initialized_t, bool init) noexcept : init_(init), 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, 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) {};

    constexpr explicit constexpr_optional_base(only_set_initialized_t, bool init) noexcept : init_(init), 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, REQUIRES(is_constructible<T, std::initializer_list<U>>)>
    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<
    std::is_trivially_destructible<T>::value,
    constexpr_optional_base<T>,
    optional_base<T>
>::type;

template <typename T>
constexpr bool is_nothrow_swappable_() {
  using std::swap;
  return noexcept(swap(std::declval<T&>(), std::declval<T&>()));
}

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_; }
  T* dataptr() {  return std::addressof(OptionalBase<T>::storage_.value_); }
  constexpr const T* dataptr() const { return static_addressof(OptionalBase<T>::storage_.value_); }

# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
  constexpr const T& contained_val() const& { return OptionalBase<T>::storage_.value_; }
  T& contained_val() & { return OptionalBase<T>::storage_.value_; }
  T&& contained_val() && { return std::move(OptionalBase<T>::storage_.value_); }
# 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 (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 (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>(only_set_initialized, rhs.initialized())
  {
    if (rhs.initialized()) new (dataptr()) T(*rhs);
  }

  optional(optional&& rhs) noexcept(std::is_nothrow_move_constructible<T>::value)
  : OptionalBase<T>(only_set_initialized, rhs.initialized())
  {
    if (rhs.initialized()) new (dataptr()) T(std::move(*rhs));
  }

  constexpr optional(const T& v) : OptionalBase<T>(v) {}

  constexpr optional(T&& v) : OptionalBase<T>(constexpr_move(v)) {}

  template <class... Args>
    constexpr explicit optional(in_place_t, Args&&... args)
        : OptionalBase<T>(in_place_t{}, constexpr_forward<Args>(args)...) {}

    template <class U, class... Args, REQUIRES(is_constructible<T, std::initializer_list<U>>)>
    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(std::is_nothrow_move_assignable<T>::value && std::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 remove_reference<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>
  optional<T>& emplace(Args&&... args)
  {
    clear();
    initialize(std::forward<Args>(args)...);
    return *this;
  }

  template <class U, class... Args>
  optional<T>& emplace(initializer_list<U> il, Args&&... args)
  {
    clear();
    initialize<U, Args...>(il, std::forward<Args>(args)...);
    return *this;
  }

  // 20.5.4.4 Swap
  void swap(optional<T>& rhs) noexcept(is_nothrow_move_constructible<T>::value && is_nothrow_swappable_<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
  constexpr T const* operator ->() const {
    return ASSERTED_EXPRESSION(initialized(), dataptr());
  }

  T* operator ->() {
    assert (initialized());
    return dataptr();
  }

  constexpr T const& operator *() const {
    return 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());
  }

  constexpr explicit operator bool() const noexcept { return initialized(); }

# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1

  template <class V>
  constexpr T value_or(V&& v) const&
  {
    return *this ? **this : static_cast<T>(constexpr_forward<V>(v));
  }

  template <class V>
  T value_or(V&& v) &&
  {
    return *this ? std::move(const_cast<optional<T>&>(*this).contained_val()) : static_cast<T>(constexpr_forward<V>(v));
  }

# else

  template <class V>
  constexpr T value_or(V&& v) const
  {
    return *this ? **this : static_cast<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(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(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;

  optional& emplace(T& v) noexcept {
    ref = static_addressof(v);
    return *this;
  }

  optional& emplace(T&&) = delete;


  void swap(optional<T&>& rhs) noexcept
  {
    std::swap(ref, rhs.ref);
  }

  // 20.5.5.3, observers
  constexpr T* operator->() const {
    return ASSERTED_EXPRESSION(ref, ref);
  }

  constexpr T& operator*() const {
    return 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 : static_cast<typename decay<T>::type>(constexpr_forward<V>(v));
  }
};


template <class T>
class optional<T&&>
{
  static_assert( sizeof(T) == 0, "optional rvalue referencs 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 optionsl<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 optionsl<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{};
    }
  };
}



# endif //___OPTIONAL_HPP___
