Make scoped enums output streamable.

base/logging.h

Index: base/logging.h
diff --git a/base/logging.h b/base/logging.h
index 8c7f20ad193eb1a6f7f398d74788e0fd95c45546..3613cd5eec580093dcb214ab30d7fc076c1218d5 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -9,6 +9,7 @@
 #include <string>
 #include <cstring>
 #include <sstream>
+#include <type_traits>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
@@ -513,12 +514,37 @@ class CheckOpResult {
 
 #endif
 
+namespace internal {
+// Type trait helper to detect scoped enums.

[danakj, 2015/11/20 21:33:58]

Can you add comments here about how unscoped enums will already convert to ints
(or their fixed type), so they won't match this rule and will "just work"?

[dcheng, 2015/11/23 18:42:13]

Done.

+template <typename T>
+using is_scoped_enum =
+    std::integral_constant<bool,
+                           std::is_enum<T>::value &&
+                               !std::is_convertible<T, int>::value>;
+}  // namespace internal
+
+template <class T,
+          typename std::enable_if<internal::is_scoped_enum<T>::value &&
+                                  std::is_signed<T>::value>::type* = nullptr>
+std::ostream& operator<<(std::ostream& os, T v) {
+  os << static_cast<intmax_t>(v);

[danakj, 2015/11/20 21:33:58]

can you mention underlying_type with a TODO here and in the unsigned one?

[dcheng, 2015/11/23 18:42:13]

Done (though I just made it a top-level comment about these two functions, to
avoid being flagged by the Department of Redundancy Department).

+  return os;
+}
+
+template <class T,
+          typename std::enable_if<internal::is_scoped_enum<T>::value &&
+                                  !std::is_signed<T>::value>::type* = nullptr>
+std::ostream& operator<<(std::ostream& os, T v) {
+  os << static_cast<uintmax_t>(v);
+  return os;
+}
+
 // Build the error message string.  This is separate from the "Impl"
 // function template because it is not performance critical and so can
 // be out of line, while the "Impl" code should be inline.  Caller
 // takes ownership of the returned string.
-template<class t1, class t2>
-std::string* MakeCheckOpString(const t1& v1, const t2& v2, const char* names) {
+template <class T1, class T2>
+std::string* MakeCheckOpString(const T1& v1, const T2& v2, const char* names) {
   std::ostringstream ss;
   ss << names << " (" << v1 << " vs. " << v2 << ")";
   std::string* msg = new std::string(ss.str());