Monty Taylor claims this helps the drizzle compile for some reason.

src/google/protobuf/extension_set.h

@@ -97,11 +97,13 @@ struct ExtensionInfo {
   bool is_repeated;
   bool is_packed;
 
-  union {
-    struct {
+  struct EnumValidityCheck {
     EnumValidityFuncWithArg* func;
     const void* arg;
-    } enum_validity_check;
+  };
+
+  union {
+    EnumValidityCheck enum_validity_check;
     const MessageLite* message_prototype;
   };