CheckedPtr support for *post*-increment and *post*-decrement operators.

This CL helps with:

1. Meeting one of the requirements that says "Ideal proposal should be
   as close to a drop-in replacement for raw pointers as possible"

2. Compiling a handful of places in Chromium that depend on these
   operators.  So far we know about:
     .../blink/renderer/core/css/parser/css_parser_token_range.h:45
     third_party/blink/renderer/platform/json/json_parser.cc:187
     third_party/blink/renderer/platform/json/json_parser.cc:85
     components/metrics/file_metrics_provider_unittest.cc:252

Bug: 1073933
Change-Id: Icb69c906dd1cfad2cc6242e81517969ff0da344c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199177Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769607}

base/memory/checked_ptr.h

@@ -159,6 +159,18 @@ class CheckedPtr {
     return *this;
   }
 
+  ALWAYS_INLINE CheckedPtr<T> operator++(int /* post_increment */) {
+    CheckedPtr<T> result = *this;
+    ++(*this);
+    return result;
+  }
+
+  ALWAYS_INLINE CheckedPtr<T> operator--(int /* post_decrement */) {
+    CheckedPtr<T> result = *this;
+    --(*this);
+    return result;
+  }
+
   ALWAYS_INLINE CheckedPtr& operator+=(ptrdiff_t delta_elems) {
     wrapped_ptr_ = Impl::Advance(wrapped_ptr_, delta_elems * sizeof(T));
     return *this;

base/memory/checked_ptr_unittest.cc

@@ -397,32 +397,49 @@ TEST(CheckedPtr, StdSwap) {
   EXPECT_EQ(g_checked_ptr_swap_cnt, 0);
 }
 
-TEST(CheckedPtr, AdvanceIntArray) {
-  // operator++
+TEST(CheckedPtr, PostIncrementOperator) {
   int foo[] = {42, 43, 44, 45};
   CheckedPtr<int> ptr = foo;
-  for (int i = 0; i < 4; ++i, ++ptr) {
-    ASSERT_EQ(*ptr, 42 + i);
+  for (int i = 0; i < 4; ++i) {
+    ASSERT_EQ(*ptr++, 42 + i);
   }
-  ptr = &foo[1];
-  for (int i = 1; i < 4; ++i, ++ptr) {
+}
+
+TEST(CheckedPtr, PostDecrementOperator) {
+  int foo[] = {42, 43, 44, 45};
+  CheckedPtr<int> ptr = &foo[3];
+  for (int i = 3; i >= 0; --i) {
+    ASSERT_EQ(*ptr--, 42 + i);
+  }
+}
+
+TEST(CheckedPtr, PreIncrementOperator) {
+  int foo[] = {42, 43, 44, 45};
+  CheckedPtr<int> ptr = foo;
+  for (int i = 0; i < 4; ++i, ++ptr) {
     ASSERT_EQ(*ptr, 42 + i);
   }
+}
 
-  // operator--
-  ptr = &foo[3];
+TEST(CheckedPtr, PreDecrementOperator) {
+  int foo[] = {42, 43, 44, 45};
+  CheckedPtr<int> ptr = &foo[3];
   for (int i = 3; i >= 0; --i, --ptr) {
     ASSERT_EQ(*ptr, 42 + i);
   }
+}
 
-  // operator+=
-  ptr = foo;
+TEST(CheckedPtr, PlusEqualsOperator) {
+  int foo[] = {42, 43, 44, 45};
+  CheckedPtr<int> ptr = foo;
   for (int i = 0; i < 4; i += 2, ptr += 2) {
     ASSERT_EQ(*ptr, 42 + i);
   }
+}
 
-  // operator-=
-  ptr = &foo[3];
+TEST(CheckedPtr, PlusMinusOperator) {
+  int foo[] = {42, 43, 44, 45};
+  CheckedPtr<int> ptr = &foo[3];
   for (int i = 3; i >= 0; i -= 2, ptr -= 2) {
     ASSERT_EQ(*ptr, 42 + i);
   }