Declare and define explicit template instantiations, since the required specializations are needed at a point where the primary template is not defined.

PiperOrigin-RevId: 616297036

src/google/protobuf/generated_message_reflection.cc

@@ -1168,6 +1168,14 @@ void Reflection::SwapFieldsImpl(
   }
 }
 
+template void Reflection::SwapFieldsImpl<true>(
+    Message* message1, Message* message2,
+    const std::vector<const FieldDescriptor*>& fields) const;
+
+template void Reflection::SwapFieldsImpl<false>(
+    Message* message1, Message* message2,
+    const std::vector<const FieldDescriptor*>& fields) const;
+
 void Reflection::SwapFields(
     Message* message1, Message* message2,
     const std::vector<const FieldDescriptor*>& fields) const {

src/google/protobuf/message.h

@@ -1308,6 +1308,14 @@ class PROTOBUF_EXPORT Reflection final {
                                              internal::ParseContext* ctx);
 };
 
+extern template void Reflection::SwapFieldsImpl<true>(
+    Message* message1, Message* message2,
+    const std::vector<const FieldDescriptor*>& fields) const;
+
+extern template void Reflection::SwapFieldsImpl<false>(
+    Message* message1, Message* message2,
+    const std::vector<const FieldDescriptor*>& fields) const;
+
 // Abstract interface for a factory for message objects.
 //
 // The thread safety for this class is implementation dependent, see comments