diff --git a/src/google/protobuf/extension_set.h b/src/google/protobuf/extension_set.h
index 3be56c4724..aba66c23a5 100644
--- a/src/google/protobuf/extension_set.h
+++ b/src/google/protobuf/extension_set.h
@@ -24,6 +24,7 @@
 #include <string>
 #include <type_traits>
 #include <utility>
+#include <variant>
 #include <vector>
 
 #include "google/protobuf/stubs/common.h"
@@ -83,6 +84,10 @@ namespace internal {
 
 class InternalMetadata;
 
+namespace v2 {
+class TableDriven;
+}  // namespace v2
+
 // Used to store values of type WireFormatLite::FieldType without having to
 // #include wire_format_lite.h.  Also, ensures that we use only one byte to
 // store these values, which is important to keep the layout of
@@ -567,6 +572,7 @@ class PROTOBUF_EXPORT ExtensionSet {
   friend class google::protobuf::internal::ReflectionVisit;
   friend struct google::protobuf::internal::DynamicExtensionInfoHelper;
   friend class google::protobuf::internal::WireFormat;
+  friend class google::protobuf::internal::v2::TableDriven;
 
   friend void internal::InitializeLazyExtensionSet();
 
@@ -645,6 +651,9 @@ class PROTOBUF_EXPORT ExtensionSet {
     virtual size_t ByteSizeLong() const = 0;
     virtual size_t SpaceUsedLong() const = 0;
 
+    virtual std::variant<size_t, const MessageLite*> UnparsedSizeOrMessage()
+        const = 0;
+
     virtual void MergeFrom(const MessageLite* prototype,
                            const LazyMessageExtension& other, Arena* arena,
                            Arena* other_arena) = 0;
diff --git a/src/google/protobuf/extension_set_heavy.cc b/src/google/protobuf/extension_set_heavy.cc
index 134d08296a..96d363e2ab 100644
--- a/src/google/protobuf/extension_set_heavy.cc
+++ b/src/google/protobuf/extension_set_heavy.cc
@@ -15,6 +15,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <initializer_list>
+#include <variant>
 #include <vector>
 
 #include "absl/base/attributes.h"