Fix build for older compilers.

If it doesn't properly do constant initialization on globals we should
not try to call virtual functions on these objects during dynamic
initialization.
Turn off the optimization in these cases.

PiperOrigin-RevId: 619251810

src/google/protobuf/extension_set.cc

@@ -154,7 +154,13 @@ void ExtensionSet::RegisterMessageExtension(const MessageLite* extendee,
              type == WireFormatLite::TYPE_GROUP);
   ExtensionInfo info(extendee, number, type, is_repeated, is_packed,
                      verify_func, is_lazy);
-  info.message_info = {prototype, prototype->GetTcParseTable()};
+  info.message_info = {prototype,
+#if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
+                       prototype->GetTcParseTable()
+#else
+                       nullptr
+#endif
+  };
   Register(info);
 }
 

src/google/protobuf/extension_set.h

@@ -140,7 +140,9 @@ struct ExtensionInfo {
 
   struct MessageInfo {
     const MessageLite* prototype;
-    // The TcParse table used for this object. Never null.
+    // The TcParse table used for this object.
+    // Never null. (except in platforms that don't constant initialize default
+    // instances)
     const internal::TcParseTableBase* tc_table;
   };