Add operator== for nullptr_t

This allows to compare wrapped pointer directly against 0, saving us trouble to extract the pointer.

Bug: 1073933
Change-Id: I071ddff9aaea723c13fcb09acb91dc4c139d7554
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2223694Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773958}

base/memory/checked_ptr.h

@@ -415,7 +415,6 @@ class CheckedPtr {
     wrapped_ptr_ = Impl::WrapRawPtr(p);
     return *this;
   }
-
   ALWAYS_INLINE CheckedPtr& operator=(std::nullptr_t) noexcept {
     wrapped_ptr_ = Impl::GetWrappedNullPtr();
     return *this;
@@ -529,6 +528,21 @@ class CheckedPtr {
   friend ALWAYS_INLINE bool operator!=(U* lhs, const CheckedPtr& rhs) {
     return rhs != lhs;  // Reverse order to call the operator above.
   }
+  // Needed for comparisons against nullptr. Without these, a slightly more
+  // costly version would be called that extracts wrapped pointer, as opposed
+  // to plain comparison against 0.
+  friend ALWAYS_INLINE bool operator==(const CheckedPtr& lhs, nullptr_t) {
+    return !lhs;
+  }
+  friend ALWAYS_INLINE bool operator!=(const CheckedPtr& lhs, nullptr_t) {
+    return !!lhs;  // Use !! otherwise the costly implicit cast will be used.
+  }
+  friend ALWAYS_INLINE bool operator==(nullptr_t, const CheckedPtr& rhs) {
+    return !rhs;
+  }
+  friend ALWAYS_INLINE bool operator!=(nullptr_t, const CheckedPtr& rhs) {
+    return !!rhs;  // Use !! otherwise the costly implicit cast will be used.
+  }
 
   friend ALWAYS_INLINE void swap(CheckedPtr& lhs, CheckedPtr& rhs) noexcept {
     Impl::IncrementSwapCountForTest();

base/memory/checked_ptr_unittest.cc

@@ -137,6 +137,7 @@ TEST_F(CheckedPtrTest, NullArrowDereference) {
 
 TEST_F(CheckedPtrTest, NullExtractNoDereference) {
   CountingCheckedPtr<int> ptr = nullptr;
+  // No dereference hence shouldn't crash.
   int* raw = ptr;
   std::ignore = raw;
   EXPECT_EQ(g_get_for_comparison_cnt, 0);
@@ -144,6 +145,28 @@ TEST_F(CheckedPtrTest, NullExtractNoDereference) {
   EXPECT_EQ(g_get_for_dereference_cnt, 0);
 }
 
+TEST_F(CheckedPtrTest, NullCmpExplicit) {
+  CountingCheckedPtr<int> ptr = nullptr;
+  EXPECT_TRUE(ptr == nullptr);
+  EXPECT_TRUE(nullptr == ptr);
+  EXPECT_FALSE(ptr != nullptr);
+  EXPECT_FALSE(nullptr != ptr);
+  // No need to unwrap pointer, just compare against 0.
+  EXPECT_EQ(g_get_for_comparison_cnt, 0);
+  EXPECT_EQ(g_get_for_extraction_cnt, 0);
+  EXPECT_EQ(g_get_for_dereference_cnt, 0);
+}
+
+TEST_F(CheckedPtrTest, NullCmpBool) {
+  CountingCheckedPtr<int> ptr = nullptr;
+  EXPECT_FALSE(ptr);
+  EXPECT_TRUE(!ptr);
+  // No need to unwrap pointer, just compare against 0.
+  EXPECT_EQ(g_get_for_comparison_cnt, 0);
+  EXPECT_EQ(g_get_for_extraction_cnt, 0);
+  EXPECT_EQ(g_get_for_dereference_cnt, 0);
+}
+
 TEST_F(CheckedPtrTest, StarDereference) {
   int foo = 42;
   CountingCheckedPtr<int> ptr = &foo;