// 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

#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

#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 tensorpipe {

// 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 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>>)>
  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(
      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;
    }
  }

  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(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, 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 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(
      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();
  }

  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

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

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 {
    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 tensorpipe

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