base::Optional: do not use T::new() but ::new.

This is also adding unit tests that check that the default ctor is not
called.

Bug: 798708
Change-Id: Ib8efb330d10517cb539fa4617e3f4fb3723d1dc0
Reviewed-on: https://chromium-review.googlesource.com/848715
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526804}

base/optional.h

@@ -134,7 +134,7 @@ class OptionalBase {
   template <class... Args>
   void Init(Args&&... args) {
     DCHECK(storage_.is_null_);
-    new (&storage_.value_) T(std::forward<Args>(args)...);
+    ::new (&storage_.value_) T(std::forward<Args>(args)...);
     storage_.is_null_ = false;
   }
 

base/optional_unittest.cc

@@ -104,6 +104,25 @@ class NonTriviallyDestructible {
   ~NonTriviallyDestructible() {}
 };
 
+class DeletedDefaultConstructor {
+ public:
+  DeletedDefaultConstructor() = delete;
+  DeletedDefaultConstructor(int foo) : foo_(foo) {}
+
+  int foo() const { return foo_; }
+
+ private:
+  int foo_;
+};
+
+class DeleteNewOperators {
+ public:
+  void* operator new(size_t) = delete;
+  void* operator new(size_t, void*) = delete;
+  void* operator new[](size_t) = delete;
+  void* operator new[](size_t, void*) = delete;
+};
+
 }  // anonymous namespace
 
 static_assert(std::is_trivially_destructible<Optional<int>>::value,
@@ -1535,4 +1554,21 @@ TEST(OptionalTest, AssignFromRValue) {
   EXPECT_EQ(1, a->move_ctors_count());
 }
 
+TEST(OptionalTest, DontCallDefaultCtor) {
+  Optional<DeletedDefaultConstructor> a;
+  EXPECT_FALSE(a.has_value());
+
+  a = base::make_optional<DeletedDefaultConstructor>(42);
+  EXPECT_TRUE(a.has_value());
+  EXPECT_EQ(42, a->foo());
+}
+
+TEST(OptionalTest, DontCallNewMemberFunction) {
+  Optional<DeleteNewOperators> a;
+  EXPECT_FALSE(a.has_value());
+
+  a = DeleteNewOperators();
+  EXPECT_TRUE(a.has_value());
+}
+
 }  // namespace base