// 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.

// Copied from https://github.com/akrzemi1/Optional (727c729dd1d9f06f225868280e50154594d7e59d)

// Modified to make it compile with exceptions disabled.

#pragma once

# include <utility>
# include <type_traits>
# include <initializer_list>
# include <cassert>
# include <functional>
# include <string>
# include <stdexcept>
# include <wtf/Assertions.h>
# include <wtf/Compiler.h>
# include <wtf/StdLibExtras.h>

# define TR2_OPTIONAL_REQUIRES(...) typename std::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

// FIXME: To make the result of value() type consistent among the compilers, we now intentionally disables move accessors.
#   define OPTIONAL_HAS_MOVE_ACCESSORS 0
// # 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

#if COMPILER_SUPPORTS(EXCEPTIONS)
#define __THROW_EXCEPTION(__exception) throw __exception;
#define __NOEXCEPT noexcept
#define __NOEXCEPT_(__exception) noexcept(__exception)
#else
#define __THROW_EXCEPTION(__exception) do { (void)__exception; CRASH(); } while (0);
#define __NOEXCEPT
#define __NOEXCEPT_(...)
#endif

namespace std {
namespace detail_ {

// NOTE: All our target compilers support is_trivially_destructible.
// // 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 COMPILER_SUPPORTS(EXCEPTIONS)
# if defined(TR2_OPTIONAL_GCC_4_7_AND_HIGHER___) || defined(TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_) || defined(TR2_OPTIONAL_MSVC_2015_AND_HIGHER___)
    // leave it; our metafunctions are already defined.
    template <typename T>
    using is_nothrow_move_constructible = std::is_nothrow_move_constructible<T>;
    template <typename T>
    using is_nothrow_move_assignable = std::is_nothrow_move_assignable<T>;
# 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
#endif

} // namespace detail_

// 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&>;

namespace detail_ {

// 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


// 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 std::logic_error {
public:
  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_(detail_::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_(detail_::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_(detail_::constexpr_move(v)) {}

    template <class... Args> explicit optional_base(in_place_t, Args&&... args)
        : init_(true), storage_(detail_::constexpr_forward<Args>(args)...) {}

    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(std::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_(detail_::constexpr_move(v)) {}

    template <class... Args> explicit constexpr constexpr_optional_base(in_place_t, Args&&... args)
      : init_(true), storage_(detail_::constexpr_forward<Args>(args)...) {}

    template <class U, class... Args, TR2_OPTIONAL_REQUIRES(std::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<
    std::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_(detail_::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>(detail_::constexpr_move(v)) {}

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

  template <class U, class... Args, TR2_OPTIONAL_REQUIRES(std::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, detail_::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_(detail_::is_nothrow_move_assignable<T>::value && detail_::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 std::enable_if
  <
    std::is_same<typename std::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(std::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_(detail_::is_nothrow_move_constructible<T>::value && __NOEXCEPT_(swap(std::declval<T&>(), std::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 ->() {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // CONSTEXPR_ASSERT(initialized());
    return dataptr();
  }

  constexpr T const& operator *() const& {
    return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
  }

  OPTIONAL_MUTABLE_CONSTEXPR T& operator *() & {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // CONSTEXPR_ASSERT(initialized());
    return contained_val();
  }

  OPTIONAL_MUTABLE_CONSTEXPR T&& operator *() && {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // CONSTEXPR_ASSERT(initialized());
    return detail_::constexpr_move(contained_val());
  }

  constexpr T const& value() const& {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
    return contained_val();
  }

  OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
    return contained_val();
  }

  OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // if (!initialized()) __THROW_EXCEPTION(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 {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
    return contained_val();
  }

  T& value() {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val());
    return 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>(detail_::constexpr_forward<V>(v));
  }

#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1

  template <class V>
  OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) &&
  {
    return *this ? detail_::constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(detail_::constexpr_forward<V>(v));
  }

#   else

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

#   endif

# else

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

# endif

};


template <class T>
class optional<T&>
{
  static_assert( !std::is_same<T, nullopt_t>::value, "bad T" );
  static_assert( !std::is_same<T, in_place_t>::value, "bad T" );
  T* ref;

public:

  // 20.5.5.1, construction/destruction
  constexpr optional() __NOEXCEPT : ref(nullptr) {}

  constexpr optional(nullopt_t) __NOEXCEPT : ref(nullptr) {}

  constexpr optional(T& v) __NOEXCEPT : ref(detail_::static_addressof(v)) {}

  optional(T&&) = delete;

  constexpr optional(const optional& rhs) __NOEXCEPT : ref(rhs.ref) {}

  explicit constexpr optional(in_place_t, T& v) __NOEXCEPT : ref(detail_::static_addressof(v)) {}

  explicit optional(in_place_t, T&&) = delete;

  ~optional() = default;

  // 20.5.5.2, mutation
  optional& operator=(nullopt_t) __NOEXCEPT {
    ref = nullptr;
    return *this;
  }

  // optional& operator=(const optional& rhs) __NOEXCEPT {
    // ref = rhs.ref;
    // return *this;
  // }

  // optional& operator=(optional&& rhs) __NOEXCEPT {
    // ref = rhs.ref;
    // return *this;
  // }

  template <typename U>
  auto operator=(U&& rhs) __NOEXCEPT
  -> typename std::enable_if
  <
    std::is_same<typename std::decay<U>::type, optional<T&>>::value,
    optional&
  >::type
  {
    ref = rhs.ref;
    return *this;
  }

  template <typename U>
  auto operator=(U&& rhs) __NOEXCEPT
  -> typename std::enable_if
  <
    !std::is_same<typename std::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 {
    // FIXME: We need to offer special assert function that can be used under the contexpr context.
    // return ref ? *ref : (throw bad_optional_access("bad optional access"), *ref);
    return *ref;
  }

  explicit constexpr operator bool() const __NOEXCEPT {
    return ref != nullptr;
  }

  template <class V>
  constexpr typename std::decay<T>::type value_or(V&& v) const
  {
    return *this ? **this : detail_::convert<typename std::decay<T>::type>(detail_::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 std::decay<T>::type> make_optional(T&& v)
{
  return optional<typename std::decay<T>::type>(detail_::constexpr_forward<T>(v));
}

template <class X>
constexpr optional<X&> make_optional(std::reference_wrapper<X> v)
{
  return optional<X&>(v.get());
}

} // namespace std

namespace WTF {

// -- WebKit Additions --
template <class OptionalType, class Callback>
ALWAYS_INLINE
auto valueOrCompute(OptionalType optional, Callback callback) -> typename OptionalType::value_type
{
    if (optional)
        return *optional;
    return callback();
}

} // namespace WTF

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

using WTF::valueOrCompute;