api: Allow v8::Maybe<MovableType>.

Change-Id: I29bcdf3302f37568e8c8925e70a01ba342c17925
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3606229
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80288}

include/v8-maybe.h

@@ -5,6 +5,9 @@
 #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)
 
@@ -57,11 +60,20 @@ class Maybe {
    * 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 {
+  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).
@@ -82,6 +94,7 @@ class Maybe {
  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_;
@@ -90,6 +103,8 @@ class Maybe {
   friend Maybe<U> Nothing();
   template <class U>
   friend Maybe<U> Just(const U& u);
+  template <class U, std::enable_if_t<!std::is_lvalue_reference_v<U>>*>
+  friend Maybe<U> Just(U&& u);
 };
 
 template <class T>
@@ -102,6 +117,14 @@ 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> {

test/unittests/BUILD.gn

@@ -216,6 +216,7 @@ v8_source_set("unittests_sources") {
     "api/isolate-unittest.cc",
     "api/remote-object-unittest.cc",
     "api/resource-constraints-unittest.cc",
+    "api/v8-maybe-unittest.cc",
     "api/v8-object-unittest.cc",
     "base/address-region-unittest.cc",
     "base/atomic-utils-unittest.cc",

test/unittests/api/v8-maybe-unittest.cc

@@ -0,0 +1,42 @@
+// Copyright 2022 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.
+
+#include "include/v8-maybe.h"
+
+#include "src/base/compiler-specific.h"
+#include "src/base/macros.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace internal {
+
+namespace {
+struct Movable {
+  Movable() = default;
+
+  Movable(const Movable&) = delete;
+  Movable& operator=(const Movable&) = delete;
+
+  Movable(Movable&&) V8_NOEXCEPT = default;
+  Movable& operator=(Movable&&) V8_NOEXCEPT = default;
+};
+}  // namespace
+
+TEST(MaybeTest, AllowMovableTypes) {
+  Maybe<Movable> m1 = Just(Movable{});
+  EXPECT_TRUE(m1.IsJust());
+
+  Maybe<Movable> m2 = Just<Movable>({});
+  EXPECT_TRUE(m2.IsJust());
+
+  Maybe<Movable> m3 = Nothing<Movable>();
+  EXPECT_TRUE(m3.IsNothing());
+
+  Maybe<Movable> m4 = Just(Movable{});
+  Movable mm = std::move(m4).FromJust();
+  USE(mm);
+}
+
+}  // namespace internal
+}  // namespace v8