Fix support of C++ references in UMA macros

This CL fixes compilation errors caused when passing const-refs or
non-const-refs as value arguments for the UMA macros.


Before this CL, an attempt to pass a const-ref as a value resulted
in errors like this:

base/metrics/histogram_macros_internal.h:30:5: error: static_assert failed due to requirement 'sizeof(const TestEnum &) == 0' "enumerator must define kMaxValue enumerat
or to use this macro!"
    static_assert(
    ^
base/metrics/histogram_macros_unittest.cc:63:3: note: in instantiation of member function 'base::internal::EnumSizeTraits<const TestEnum &, void>::Count' requested here
  UMA_HISTOGRAM_ENUMERATION("Test.ScopedEnumeration3", value_ref);
  ^
base/metrics/histogram_macros.h:84:7: note: expanded from macro 'UMA_HISTOGRAM_ENUMERATION'
      INTERNAL_UMA_HISTOGRAM_ENUMERATION_DEDUCE_BOUNDARY)               \
      ^
In file included from ../../base/metrics/histogram_macros_unittest.cc:5:
In file included from ../../base/metrics/histogram_macros.h:10:
base/metrics/histogram_macros_internal.h:33:12: error: reference to type 'const TestEnum' requires an initializer
    return Enum();


Bug: none
Change-Id: I875ff783bda12cc30079bcf6188ff843b9b55a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729593Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682898}

base/metrics/histogram_macros_internal.h

@@ -145,9 +145,9 @@ struct EnumSizeTraits<
 #define INTERNAL_HISTOGRAM_EXACT_LINEAR_WITH_FLAG(name, sample, boundary,  \
                                                   flag)                    \
   do {                                                                     \
-    static_assert(!std::is_enum<decltype(sample)>::value,                  \
+    static_assert(!std::is_enum<std::decay_t<decltype(sample)>>::value,    \
                   "|sample| should not be an enum type!");                 \
-    static_assert(!std::is_enum<decltype(boundary)>::value,                \
+    static_assert(!std::is_enum<std::decay_t<decltype(boundary)>>::value,  \
                   "|boundary| should not be an enum type!");               \
     STATIC_HISTOGRAM_POINTER_BLOCK(                                        \
         name, Add(sample),                                                 \
@@ -161,9 +161,9 @@ struct EnumSizeTraits<
 #define INTERNAL_HISTOGRAM_SCALED_EXACT_LINEAR_WITH_FLAG(                      \
     name, sample, count, boundary, scale, flag)                                \
   do {                                                                         \
-    static_assert(!std::is_enum<decltype(sample)>::value,                      \
+    static_assert(!std::is_enum<std::decay_t<decltype(sample)>>::value,        \
                   "|sample| should not be an enum type!");                     \
-    static_assert(!std::is_enum<decltype(boundary)>::value,                    \
+    static_assert(!std::is_enum<std::decay_t<decltype(boundary)>>::value,      \
                   "|boundary| should not be an enum type!");                   \
     class ScaledLinearHistogramInstance : public base::ScaledLinearHistogram { \
      public:                                                                   \
@@ -186,7 +186,8 @@ struct EnumSizeTraits<
 #define INTERNAL_UMA_HISTOGRAM_ENUMERATION_DEDUCE_BOUNDARY(name, sample,       \
                                                            flags)              \
   INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                                    \
-      name, sample, base::internal::EnumSizeTraits<decltype(sample)>::Count(), \
+      name, sample,                                                            \
+      base::internal::EnumSizeTraits<std::decay_t<decltype(sample)>>::Count(), \
       flags)
 
 // Note: The value in |sample| must be strictly less than |enum_size|.
@@ -231,8 +232,8 @@ struct EnumSizeTraits<
     using decayed_sample = std::decay<decltype(sample)>::type;               \
     static_assert(std::is_enum<decayed_sample>::value,                       \
                   "Unexpected: |sample| is not at enum.");                   \
-    constexpr auto boundary =                                                \
-        base::internal::EnumSizeTraits<decltype(sample)>::Count();           \
+    constexpr auto boundary = base::internal::EnumSizeTraits<                \
+        std::decay_t<decltype(sample)>>::Count();                            \
     static_assert(                                                           \
         static_cast<uintmax_t>(boundary) <                                   \
             static_cast<uintmax_t>(                                          \

base/metrics/histogram_macros_unittest.cc

@@ -20,7 +20,7 @@ TEST(ScopedHistogramTimer, TwoTimersOneScope) {
 // - integral types
 // - unscoped enums
 // - scoped enums
-TEST(HistogramMacro, IntegralPsuedoEnumeration) {
+TEST(HistogramMacro, IntegralPseudoEnumeration) {
   UMA_HISTOGRAM_ENUMERATION("Test.FauxEnumeration", 1, 1000);
 }
 
@@ -54,4 +54,20 @@ TEST(HistogramMacro, ScopedEnumeration) {
                             TestEnum2::MAX_ENTRIES);
 }
 
+// Compile tests for UMA_HISTOGRAM_ENUMERATION when the value type is:
+// - a const reference to an enum
+// - a non-const reference to an enum
+TEST(HistogramMacro, EnumerationConstRef) {
+  enum class TestEnum { kValue, kMaxValue = kValue };
+  const TestEnum& value_ref = TestEnum::kValue;
+  UMA_HISTOGRAM_ENUMERATION("Test.ScopedEnumeration3", value_ref);
+}
+
+TEST(HistogramMacro, EnumerationNonConstRef) {
+  enum class TestEnum { kValue, kMaxValue = kValue };
+  TestEnum value = TestEnum::kValue;
+  TestEnum& value_ref = value;
+  UMA_HISTOGRAM_ENUMERATION("Test.ScopedEnumeration4", value_ref);
+}
+
 }  // namespace base