Add message type traits that can be declared in duplicate so that .proto.h can

src/google/protobuf/compiler/cpp/file.cc

@@ -1460,20 +1460,35 @@ class FileGenerator::ForwardDeclarations {
     if (ShouldGenerateExternSpecializations(options)) {
       for (const auto& c : classes_) {
         if (!ShouldGenerateClass(c.second, options)) continue;
+        auto vars = p->WithVars(
+            {{"class", QualifiedClassName(c.second, options)},
+             {"default_name", QualifiedDefaultInstanceName(c.second, options,
+                                                           /*split=*/false)}});
         // To reduce total linker input size in large binaries we make these
         // functions extern and define then in the pb.cc file. This avoids bloat
         // in callers by having duplicate definitions of the template.
         // However, it increases the size of the pb.cc translation units so it
         // is a tradeoff.
-        p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
+        p->Emit(R"cc(
           extern template void* Arena::DefaultConstruct<$class$>(Arena*);
         )cc");
         if (!IsMapEntryMessage(c.second)) {
-          p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
+          p->Emit(R"cc(
             extern template void* Arena::CopyConstruct<$class$>(Arena*,
                                                                 const void*);
           )cc");
         }
+        // We can't make a constexpr pointer to the global if we have DLL
+        // linkage so skip this.
+        // The fallback traits are slower, but correct.
+        if (options.dllexport_decl.empty()) {
+          p->Emit(R"cc(
+            template <>
+            internal::GeneratedMessageTraitsT<decltype($default_name$),
+                                              &$default_name$>
+                internal::MessageTraitsImpl::value<$class$>;
+          )cc");
+        }
       }
     }
   }

src/google/protobuf/compiler/cpp/message.cc

@@ -2041,18 +2041,6 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
                                                $pbi$::ParseContext* ctx);
           )cc");
         }},
-       {"raw_default_instance",
-        [&] {
-          // We can't make a constexpr pointer to the global if we have DLL
-          // linkage so skip this. The fallback in
-          // `MessageLite::GetStrongPointerForType` will do the right thing in
-          // those platforms.
-          if (!options_.dllexport_decl.empty()) return;
-          p->Emit(R"cc(
-            static constexpr const void* _raw_default_instance_ =
-                &_$classname$_default_instance_;
-          )cc");
-        }},
        {"decl_impl", [&] { GenerateImplDefinition(p); }},
        {"classdata_type",
         HasDescriptorMethods(descriptor_->file(), options_)
@@ -2210,8 +2198,6 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
           $decl_data$;
           $post_loop_handler$;
 
-          $raw_default_instance$;
-
           friend class ::$proto_ns$::MessageLite;
           friend class ::$proto_ns$::Arena;
           template <typename T>