Reduce the cost of CheckTypeAndMergeFrom in LITE_RUNTIME objects when icf is

enabled.
DownCastToGenerated injects the strong reference to T, which makes all instances of CheckTypeAndMergeFrom unique and not candidates for identical code folding.
The strong reference is not needed here in the member function.

PiperOrigin-RevId: 634033477

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

@@ -3744,11 +3744,14 @@ void MessageGenerator::GenerateMergeFrom(io::Printer* p) {
 
   if (!HasDescriptorMethods(descriptor_->file(), options_)) {
     // Generate CheckTypeAndMergeFrom().
-    format(
-        "void $classname$::CheckTypeAndMergeFrom(\n"
-        "    const ::$proto_ns$::MessageLite& from) {\n"
-        "  MergeFrom(::$proto_ns$::DownCastToGenerated<$classname$>(from));\n"
-        "}\n");
+    // We pass `*this` instead of `default_instance()` to allow for ICF.
+    p->Emit(R"cc(
+      void $classname$::CheckTypeAndMergeFrom(
+          const ::$proto_ns$::MessageLite& from) {
+        $pbi$::AssertDownCast(from, *this);
+        MergeFrom(static_cast<const $classname$&>(from));
+      }
+    )cc");
   }
 }
 

src/google/protobuf/message_lite.h

@@ -863,6 +863,11 @@ T* OnShutdownDelete(T* p) {
   return p;
 }
 
+inline void AssertDownCast(const MessageLite& from, const MessageLite& to) {
+  ABSL_DCHECK(internal::TypeId::Get(from) == internal::TypeId::Get(to))
+      << "Cannot downcast " << from.GetTypeName() << " to " << to.GetTypeName();
+}
+
 }  // namespace internal
 
 std::string ShortFormat(const MessageLite& message_lite);