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Editing: v8-maybe.h

// Copyright 2021 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef INCLUDE_V8_MAYBE_H_
#define INCLUDE_V8_MAYBE_H_

#include <type_traits>
#include <utility>

#include "v8-internal.h"  // NOLINT(build/include_directory)
#include "v8config.h"     // NOLINT(build/include_directory)

namespace v8 {

namespace api_internal {
// Called when ToChecked is called on an empty Maybe.
V8_EXPORT void FromJustIsNothing();
}  // namespace api_internal

/**
 * A simple Maybe type, representing an object which may or may not have a
 * value, see https://hackage.haskell.org/package/base/docs/Data-Maybe.html.
 *
 * If an API method returns a Maybe<>, the API method can potentially fail
 * either because an exception is thrown, or because an exception is pending,
 * e.g. because a previous API call threw an exception that hasn't been caught
 * yet, or because a TerminateExecution exception was thrown. In that case, a
 * "Nothing" value is returned.
 */
template <class T>
class Maybe {
 public:
 V8_INLINE bool IsNothing() const { return !has_value_; }
 V8_INLINE bool IsJust() const { return has_value_; }

/**
 * An alias for |FromJust|. Will crash if the Maybe<> is nothing.
 */
 V8_INLINE T ToChecked() const { return FromJust(); }

/**
   * Short-hand for ToChecked(), which doesn't return a value. To be used, where
   * the actual value of the Maybe is not needed like Object::Set.
   */
  V8_INLINE void Check() const {
    if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing();
  }

/**
 * Converts this Maybe<> to a value of type T. If this Maybe<> is
 * nothing (empty), |false| is returned and |out| is left untouched.
 */
 V8_WARN_UNUSED_RESULT V8_INLINE bool To(T* out) const {
 if (V8_LIKELY(IsJust())) *out = value_;
 return IsJust();
 }

/**
 * Converts this Maybe<> to a value of type T. If this Maybe<> is
 * nothing (empty), V8 will crash the process.
 */
 V8_INLINE T FromJust() const& {
 if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing();
 return value_;
 }

/**
 * Converts this Maybe<> to a value of type T. If this Maybe<> is
 * nothing (empty), V8 will crash the process.
 */
 V8_INLINE T FromJust() && {
 if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing();
 return std::move(value_);
 }

/**
 * Converts this Maybe<> to a value of type T, using a default value if this
 * Maybe<> is nothing (empty).
 */
 V8_INLINE T FromMaybe(const T& default_value) const {
 return has_value_ ? value_ : default_value;
 }

V8_INLINE bool operator==(const Maybe& other) const {
    return (IsJust() == other.IsJust()) &&
           (!IsJust() || FromJust() == other.FromJust());
  }

V8_INLINE bool operator!=(const Maybe& other) const {
    return !operator==(other);
  }

private:
  Maybe() : has_value_(false) {}
  explicit Maybe(const T& t) : has_value_(true), value_(t) {}
  explicit Maybe(T&& t) : has_value_(true), value_(std::move(t)) {}

bool has_value_;
  T value_;

template <class U>
 friend Maybe Nothing();
 template <class U>
 friend Maybe Just(const U& u);
 template <class U, std::enable_if_t<!std::is_lvalue_reference_v>*>
 friend Maybe Just(U&& u);
};

template <class T>
inline Maybe<T> Nothing() {
 return Maybe<T>();
}

template <class T>
inline Maybe<T> Just(const T& t) {
 return Maybe<T>(t);
}

// Don't use forwarding references here but instead use two overloads.
// Forwarding references only work when type deduction takes place, which is not
// the case for callsites such as Just<Type>(t).
template <class T, std::enable_if_t<!std::is_lvalue_reference_v<T>>* = nullptr>
inline Maybe<T> Just(T&& t) {
 return Maybe<T>(std::move(t));
}

// A template specialization of Maybe<T> for the case of T = void.
template <>
class Maybe<void> {
 public:
 V8_INLINE bool IsNothing() const { return !is_valid_; }
 V8_INLINE bool IsJust() const { return is_valid_; }

V8_INLINE bool operator==(const Maybe& other) const {
    return IsJust() == other.IsJust();
  }

V8_INLINE bool operator!=(const Maybe& other) const {
    return !operator==(other);
  }

private:
  struct JustTag {};

Maybe() : is_valid_(false) {}
  explicit Maybe(JustTag) : is_valid_(true) {}

bool is_valid_;

template <class U>
 friend Maybe Nothing();
 friend Maybe<void> JustVoid();
};

inline Maybe<void> JustVoid() { return Maybe<void>(Maybe<void>::JustTag()); }

}  // namespace v8

#endif  // INCLUDE_V8_MAYBE_H_

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