Optional.h

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// 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
//
// From https://github.com/akrzemi1/Optional
//
// C10
// - Move to `c10` namespace.
// - Remove macro use in line 478 because the nvcc device compiler cannot handle
// it.
// - revise constructor logic so that it is consistent with c++ 17 standard
// documented here in (8):
// https://en.cppreference.com/w/cpp/utility/optional/optional, and could be
// able to support initialization of optionals from convertible type U, also
// remove two old constructors optional(const T&) and optional(T&&) as it could
// be handled by the template<U=T> case with default template argument.
// - `constexpr struct in_place_t {} in_place{}` is moved to
// `c10/util/in_place.h`, so that it can also be used in `c10/util/variant.h`.
// - Remove special cases for pre-c++14 compilers to make code simpler
//
//
// Open3D
// - Namespace change: open3d::utility::optional

#pragma once

#include <cassert>
#include <functional>
#include <initializer_list>
#include <stdexcept>
#include <string>
#include <type_traits>
#include <utility>

#define TR2_OPTIONAL_REQUIRES(...) \
    typename std::enable_if<__VA_ARGS__::value, bool>::type = false

namespace open3d {
namespace utility {

struct in_place_t {
    explicit in_place_t() = default;
};

constexpr in_place_t in_place{};

// 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

#if defined(__CUDA_ARCH__)
#define TR2_OPTIONAL_HOST_CONSTEXPR
#else
#define TR2_OPTIONAL_HOST_CONSTEXPR constexpr
#endif

namespace detail_ {

// VS doesn't handle constexpr well, so we need to skip these stuff.
#if (defined _MSC_VER)
template <typename T>
T* static_addressof(T& ref) {
    return std::addressof(ref);
}
#else
// 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);
}
#endif

// 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{};

// 20.5.7, 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)
        : 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(
                      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_(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(
                      std::is_constructible<T, std::initializer_list<U>>)>
    constexpr 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> {
    template <class U>  // re-declaration for nvcc on Windows.
    using OptionalBase = typename std::conditional<
            std::is_trivially_destructible<U>::value,  // if possible
            constexpr_optional_base<typename std::remove_const<
                    U>::type>,  // use base with trivial destructor
            optional_base<typename std::remove_const<U>::type>>::type;

    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_);
    }

    constexpr const T& contained_val() const& {
        return OptionalBase<T>::storage_.value_;
    }
    constexpr T&& contained_val() && {
        return std::move(OptionalBase<T>::storage_.value_);
    }
    constexpr T& contained_val() & { return OptionalBase<T>::storage_.value_; }

    void clear() noexcept {
        if (initialized()) dataptr()->~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(
            std::is_nothrow_move_constructible<T>::value)
        : OptionalBase<T>() {
        if (rhs.initialized()) {
            ::new (static_cast<void*>(dataptr())) T(std::move(*rhs));
            OptionalBase<T>::init_ = true;
        }
    }

    // see https://github.com/akrzemi1/Optional/issues/16
    // and https://en.cppreference.com/w/cpp/utility/optional/optional,
    // in constructor 8, the std::optional spec can allow initialization
    // of optionals from convertible type U
    //
    // 8 - implicit move construct from value
    template <typename U = T,
              TR2_OPTIONAL_REQUIRES(std::is_constructible<T, U&&>::value &&
                                    !std::is_same<typename std::decay<U>::type,
                                                  in_place_t>::value &&
                                    !std::is_same<typename std::decay<U>::type,
                                                  optional<T>>::value &&
                                    std::is_convertible<U&&, T>)>
    constexpr optional(U&& u) : OptionalBase<T>(std::forward<U>(u)) {}

    // 8 - explicit move construct from value
    template <typename U = T,
              TR2_OPTIONAL_REQUIRES(std::is_constructible<T, U&&>::value &&
                                    !std::is_same<typename std::decay<U>::type,
                                                  in_place_t>::value &&
                                    !std::is_same<typename std::decay<U>::type,
                                                  optional<T>>::value &&
                                    !std::is_convertible<U&&, T>)>
    explicit constexpr optional(U&& u) : OptionalBase<T>(std::forward<U>(u)) {}

    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(
                      std::is_constructible<T, std::initializer_list<U>>)>
    constexpr 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 = T>
    auto operator=(U&& v) -> typename std::enable_if<
            std::is_constructible<T, U>::value &&
                    !std::is_same<typename std::decay<U>::type,
                                  optional<T>>::value &&
                    (std::is_scalar<T>::value ||
                     std::is_same<typename std::decay<U>::type, T>::value) &&
                    std::is_assignable<T&, U>::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(
            std::is_nothrow_move_constructible<T>::value&& noexcept(
                    std::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 bool has_value() const noexcept { return initialized(); }

    TR2_OPTIONAL_HOST_CONSTEXPR T const* operator->() const {
        return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T* operator->() {
        assert(initialized());
        return dataptr();
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T const& operator*() const& {
        return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T& operator*() & {
        assert(initialized());
        return contained_val();
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T&& operator*() && {
        assert(initialized());
        return constexpr_move(contained_val());
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T const& value() const& {
        return initialized()
                       ? contained_val()
                       : (throw bad_optional_access("bad optional access"),
                          contained_val());
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T& value() & {
        return initialized()
                       ? contained_val()
                       : (throw bad_optional_access("bad optional access"),
                          contained_val());
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T&& value() && {
        if (!initialized()) throw bad_optional_access("bad optional access");
        return std::move(contained_val());
    }

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

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

    // 20.6.3.6, modifiers
    void reset() noexcept { clear(); }
};

// XXX: please refrain from using optional<T&>, since it is being against with
// the optional standard in c++ 17, see the debate and the details here:
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3406#rationale.refs
// if you need it, consider using optional<std::reference_wrapper<T>> or *
// pointer
//
// we leave the implementation here in case we want to reconsider using it in
// the future if it becomes a definitely necessary case.
template <class T>
class optional<T&> {
    // add this assert to prevent user from using optional reference as
    // indicated above
    static_assert(sizeof(T) == 0,
                  "optional references is ill-formed, \
    consider use optional of a std::reference_wrapper of type T to \
    hold a reference if you really need to");

    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) {}

    template <typename U = T>
    constexpr optional(U& u) noexcept : ref(detail_::static_addressof(u)) {}

    template <typename U = T>
    optional(U&&) = 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
    TR2_OPTIONAL_HOST_CONSTEXPR T* operator->() const {
        return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
    }

    TR2_OPTIONAL_HOST_CONSTEXPR T& operator*() const {
        return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
    }

    constexpr T& value() const {
        return ref ? *ref
                   : (throw bad_optional_access("bad optional access"), *ref);
    }

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

    constexpr bool has_value() 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>(
                               constexpr_forward<V>(v));
    }

    // x.x.x.x, modifiers
    void reset() noexcept { ref = nullptr; }
};

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>(constexpr_forward<T>(v));
}

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

}  // namespace utility
}  // namespace open3d

namespace std {
template <typename T>
struct hash<open3d::utility::optional<T>> {
    typedef typename hash<T>::result_type result_type;
    typedef open3d::utility::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<open3d::utility::optional<T&>> {
    typedef typename hash<T>::result_type result_type;
    typedef open3d::utility::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
#undef TR2_OPTIONAL_HOST_CONSTEXPR