Indicate if a message is a DynamicMessage in the class data and improve the

error message from Dynamic/DownCastMessage when trying to downcast a
DynamicMessage to a generated type.
Previously the error message would say "can't down cast Foo to Foo", which is
confusing.

PiperOrigin-RevId: 647363909

src/google/protobuf/dynamic_message.cc

@@ -666,6 +666,7 @@ const Message* DynamicMessageFactory::GetPrototypeNoLock(
   *target = type_info;
 
   type_info->class_data.descriptor = type;
+  type_info->class_data.is_dynamic = true;
   type_info->pool = (pool_ == nullptr) ? type->file()->pool() : pool_;
   type_info->factory = this;
 

src/google/protobuf/message_lite.cc

@@ -195,6 +195,17 @@ void MessageLite::LogInitializationErrorMessage() const {
 
 namespace internal {
 
+void FailDynamicCast(const MessageLite& from, const MessageLite& to) {
+  const auto to_name = to.GetTypeName();
+  if (internal::GetClassData(from)->is_dynamic) {
+    ABSL_LOG(FATAL)
+        << "Cannot downcast from a DynamicMessage to generated type "
+        << to_name;
+  }
+  const auto from_name = from.GetTypeName();
+  ABSL_LOG(FATAL) << "Cannot downcast " << from_name << " to " << to_name;
+}
+
 template <bool aliasing>
 bool MergeFromImpl(absl::string_view input, MessageLite* msg,
                    const internal::TcParseTableBase* tc_table,

src/google/protobuf/message_lite.h

@@ -649,6 +649,7 @@ class PROTOBUF_EXPORT MessageLite {
     // LITE objects (ie !descriptor_methods) collocate their name as a
     // char[] just beyond the ClassData.
     bool is_lite;
+    bool is_dynamic = false;
 
     // In normal mode we have the small constructor to avoid the cost in
     // codegen.
@@ -1087,12 +1088,19 @@ T* DynamicCastMessage(MessageLite* from) {
       DynamicCastMessage<T>(static_cast<const MessageLite*>(from)));
 }
 
+namespace internal {
+[[noreturn]] PROTOBUF_EXPORT void FailDynamicCast(const MessageLite& from,
+                                                  const MessageLite& to);
+}  // namespace internal
+
 template <typename T>
 const T& DynamicCastMessage(const MessageLite& from) {
   const T* destination_message = DynamicCastMessage<T>(&from);
-  ABSL_CHECK(destination_message != nullptr)
+  if (ABSL_PREDICT_FALSE(destination_message == nullptr)) {
-      << "Cannot downcast " << from.GetTypeName() << " to "
+    // Move the logging into an out-of-line function to reduce bloat in the
-      << T::default_instance().GetTypeName();
+    // caller.
+    internal::FailDynamicCast(from, T::default_instance());
+  }
   return *destination_message;
 }