optional.cpp

/*

  From https://github.com/akrzemi1/Optional. 
  Modifications by Asher Haig ([email protected]) for constexpr&.

  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

  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.

*/

# ifndef ___OPTIONAL_HPP___
# define ___OPTIONAL_HPP___

# 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 Optional {
#define NDEBUG 1

using namespace ::std;

// 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(const T& ref)
{
  return const_cast<T*>( &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
struct bad_Optional_access : std::logic_error {
  explicit bad_Optional_access(const std::string& what_arg) : std::logic_error{what_arg} {}
  explicit bad_Optional_access(const char* what_arg) : std::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>() {};

  constexpr 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(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 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
  
  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(const 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);
  }
  
  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<const T&>
: public Optional<T&>
{
  using Optional<T&>::Optional;
};


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(std::reference_wrapper<X> v)
{
  return Optional<X&>(v.get());
}


} // namespace Optional

namespace std
{
  template <typename T>
  struct hash<Optional<T>>
  {
    typedef typename hash<T>::result_type result_type;
    typedef 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<Optional<T&>>
  {
    typedef typename hash<T>::result_type result_type;
    typedef 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___