base: Support using (D)CHECK on scoped enums.

base/template_util.h

Index: base/template_util.h
diff --git a/base/template_util.h b/base/template_util.h
index 74c8e5afdc63dfb63f25922fc62835d854a8d48b..98506f029dee2f22a31fba0d2999b8de33cd72d3 100644
--- a/base/template_util.h
+++ b/base/template_util.h
@@ -6,6 +6,7 @@
 #define BASE_TEMPLATE_UTIL_H_
 
 #include <stddef.h>
+#include <iosfwd>
 #include <type_traits>
 #include <utility>
 
@@ -57,6 +58,15 @@ struct IsAssignableImpl
 template <class Lvalue, class Rvalue>
 struct IsAssignableImpl<Lvalue, Rvalue, true> : public std::false_type {};
 
+// Uses expression SFINAE to detect whether using operator<< would work.
+template <typename T, typename = void>
+struct SupportsOstreamOperator : std::false_type {};
+template <typename T>
+struct SupportsOstreamOperator<T,
+                               decltype(void(std::declval<std::ostream&>()
+                                             << std::declval<T>()))>
+    : std::true_type {};
+
 }  // namespace internal
 
 // TODO(crbug.com/554293): Remove this when all platforms have this in the std
@@ -82,6 +92,26 @@ struct is_move_assignable
                            const typename std::add_rvalue_reference<T>::type> {
 };
 
+// underlying_type produces the integer type backing an enum type.
+// This hacks around libstdc++ 4.6 missing std::underlying_type, while we need
+// to support it.
+// TODO(crbug.com/554293): Remove this when all platforms have this in the std
+// namespace.
+#define CR_GLIBCXX_4_7_0 20120322
+#define CR_GLIBCXX_4_5_4 20120702
+#define CR_GLIBCXX_4_6_4 20121127
+#if defined(__GLIBCXX__) &&                                               \
+    (__GLIBCXX__ < CR_GLIBCXX_4_7_0 || __GLIBCXX__ == CR_GLIBCXX_4_5_4 || \
+     __GLIBCXX__ == CR_GLIBCXX_4_6_4)
+template <typename T>
+struct underlying_type {
+  using type = __underlying_type(T);
+};
+#else
+template <typename T>
+using underlying_type = std::underlying_type<T>;
+#endif
+
 }  // namespace base
 
 #endif  // BASE_TEMPLATE_UTIL_H_