// 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 <valijson/exceptions.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 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, constexpr_optional_base<typename std::remove_const<T>::type>,
                              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() : (valijson::throwRuntimeError("bad optional access"), contained_val());
    }

    OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
        return initialized() ? contained_val() : (valijson::throwRuntimeError("bad optional access"), contained_val());
    }

    OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
        if (!initialized()) valijson::throwRuntimeError("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() : (valijson::throwRuntimeError("bad optional access"), contained_val());
    }

    T& value() { return initialized() ? contained_val() : (valijson::throwRuntimeError("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;
    }

    // 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 : (valijson::throwRuntimeError("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 std

#undef TR2_OPTIONAL_REQUIRES
#undef TR2_OPTIONAL_ASSERTED_EXPRESSION

#endif  //___OPTIONAL_HPP___