Implement conditional copy/move ctors/assign-operators.

BUG=784732
TEST=Ran trybot.

Change-Id: Iec5f9eaa7482d4e23f5bf2eea4b34c9cd867f89d
Reviewed-on: https://chromium-review.googlesource.com/856021
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532004}

base/optional.h

@@ -265,6 +265,58 @@ class OptionalBase {
   OptionalStorage<T> storage_;
 };
 
+// The following {Copy,Move}{Constructible,Assignable} structs are helpers to
+// implement constructor/assign-operator overloading. Specifically, if T is
+// is not movable but copyable, Optional<T>'s move constructor should not
+// participate in overload resolution. This inheritance trick implements that.
+template <bool is_copy_constructible>
+struct CopyConstructible {};
+
+template <>
+struct CopyConstructible<false> {
+  constexpr CopyConstructible() = default;
+  constexpr CopyConstructible(const CopyConstructible&) = delete;
+  constexpr CopyConstructible(CopyConstructible&&) = default;
+  CopyConstructible& operator=(const CopyConstructible&) = default;
+  CopyConstructible& operator=(CopyConstructible&&) = default;
+};
+
+template <bool is_move_constructible>
+struct MoveConstructible {};
+
+template <>
+struct MoveConstructible<false> {
+  constexpr MoveConstructible() = default;
+  constexpr MoveConstructible(const MoveConstructible&) = default;
+  constexpr MoveConstructible(MoveConstructible&&) = delete;
+  MoveConstructible& operator=(const MoveConstructible&) = default;
+  MoveConstructible& operator=(MoveConstructible&&) = default;
+};
+
+template <bool is_copy_assignable>
+struct CopyAssignable {};
+
+template <>
+struct CopyAssignable<false> {
+  constexpr CopyAssignable() = default;
+  constexpr CopyAssignable(const CopyAssignable&) = default;
+  constexpr CopyAssignable(CopyAssignable&&) = default;
+  CopyAssignable& operator=(const CopyAssignable&) = delete;
+  CopyAssignable& operator=(CopyAssignable&&) = default;
+};
+
+template <bool is_move_assignable>
+struct MoveAssignable {};
+
+template <>
+struct MoveAssignable<false> {
+  constexpr MoveAssignable() = default;
+  constexpr MoveAssignable(const MoveAssignable&) = default;
+  constexpr MoveAssignable(MoveAssignable&&) = default;
+  MoveAssignable& operator=(const MoveAssignable&) = default;
+  MoveAssignable& operator=(MoveAssignable&&) = delete;
+};
+
 }  // namespace internal
 
 // base::Optional is a Chromium version of the C++17 optional class:
@@ -279,12 +331,18 @@ class OptionalBase {
 // - No exceptions are thrown, because they are banned from Chromium.
 // - All the non-members are in the 'base' namespace instead of 'std'.
 template <typename T>
-class Optional : public internal::OptionalBase<T> {
+class Optional
+    : public internal::OptionalBase<T>,
+      public internal::CopyConstructible<std::is_copy_constructible<T>::value>,
+      public internal::MoveConstructible<std::is_move_constructible<T>::value>,
+      public internal::CopyAssignable<std::is_copy_constructible<T>::value &&
+                                      std::is_copy_assignable<T>::value>,
+      public internal::MoveAssignable<std::is_move_constructible<T>::value &&
+                                      std::is_move_assignable<T>::value> {
  public:
   using value_type = T;
 
   // Defer default/copy/move constructor implementation to OptionalBase.
-  // TODO(hidehiko): Implement conditional enabling.
   constexpr Optional() = default;
   constexpr Optional(const Optional& other) = default;
   constexpr Optional(Optional&& other) = default;
@@ -315,7 +373,6 @@ class Optional : public internal::OptionalBase<T> {
   ~Optional() = default;
 
   // Defer copy-/move- assign operator implementation to OptionalBase.
-  // TOOD(hidehiko): Implement conditional enabling.
   Optional& operator=(const Optional& other) = default;
   Optional& operator=(Optional&& other) = default;
 

base/optional_unittest.cc

@@ -115,11 +115,29 @@ class DeletedDefaultConstructor {
   int foo_;
 };
 
-class DeletedCopyConstructor {
+class DeletedCopy {
  public:
-  explicit DeletedCopyConstructor(int foo) : foo_(foo) {}
-  DeletedCopyConstructor(const DeletedCopyConstructor&) = delete;
-  DeletedCopyConstructor(DeletedCopyConstructor&&) = default;
+  explicit DeletedCopy(int foo) : foo_(foo) {}
+  DeletedCopy(const DeletedCopy&) = delete;
+  DeletedCopy(DeletedCopy&&) = default;
+
+  DeletedCopy& operator=(const DeletedCopy&) = delete;
+  DeletedCopy& operator=(DeletedCopy&&) = default;
+
+  int foo() const { return foo_; }
+
+ private:
+  int foo_;
+};
+
+class DeletedMove {
+ public:
+  explicit DeletedMove(int foo) : foo_(foo) {}
+  DeletedMove(const DeletedMove&) = default;
+  DeletedMove(DeletedMove&&) = delete;
+
+  DeletedMove& operator=(const DeletedMove&) = default;
+  DeletedMove& operator=(DeletedMove&&) = delete;
 
   int foo() const { return foo_; }
 
@@ -279,8 +297,18 @@ TEST(OptionalTest, MoveConstructor) {
   // Even if copy constructor is deleted, move constructor needs to work.
   // Note that it couldn't be constexpr.
   {
-    Optional<DeletedCopyConstructor> first(in_place, 42);
-    Optional<DeletedCopyConstructor> second(std::move(first));
+    Optional<DeletedCopy> first(in_place, 42);
+    Optional<DeletedCopy> second(std::move(first));
+
+    EXPECT_TRUE(second.has_value());
+    EXPECT_EQ(42, second->foo());
+
+    EXPECT_TRUE(first.has_value());
+  }
+
+  {
+    Optional<DeletedMove> first(in_place, 42);
+    Optional<DeletedMove> second(std::move(first));
 
     EXPECT_TRUE(second.has_value());
     EXPECT_EQ(42, second->foo());
@@ -465,6 +493,26 @@ TEST(OptionalTest, AssignObject) {
     EXPECT_TRUE(a.value() == TestObject(3, 0.1));
     EXPECT_TRUE(a == b);
   }
+
+  {
+    Optional<DeletedMove> a(in_place, 42);
+    Optional<DeletedMove> b;
+    b = a;
+
+    EXPECT_TRUE(!!a);
+    EXPECT_TRUE(!!b);
+    EXPECT_EQ(a->foo(), b->foo());
+  }
+
+  {
+    Optional<DeletedMove> a(in_place, 42);
+    Optional<DeletedMove> b(in_place, 1);
+    b = a;
+
+    EXPECT_TRUE(!!a);
+    EXPECT_TRUE(!!b);
+    EXPECT_EQ(a->foo(), b->foo());
+  }
 }
 
 TEST(OptionalTest, AssignObject_rvalue) {
@@ -513,6 +561,26 @@ TEST(OptionalTest, AssignObject_rvalue) {
     EXPECT_EQ(TestObject::State::MOVE_ASSIGNED, a->state());
     EXPECT_EQ(TestObject::State::MOVED_FROM, b->state());
   }
+
+  {
+    Optional<DeletedMove> a(in_place, 42);
+    Optional<DeletedMove> b;
+    b = std::move(a);
+
+    EXPECT_TRUE(!!a);
+    EXPECT_TRUE(!!b);
+    EXPECT_EQ(42, b->foo());
+  }
+
+  {
+    Optional<DeletedMove> a(in_place, 42);
+    Optional<DeletedMove> b(in_place, 1);
+    b = std::move(a);
+
+    EXPECT_TRUE(!!a);
+    EXPECT_TRUE(!!b);
+    EXPECT_EQ(42, b->foo());
+  }
 }
 
 TEST(OptionalTest, AssignNull) {