Fix generation of HasExtension and ExtensionSize functions to restric…

…t to the

right kind of extension. This way you get a compile time failure instead of a
runtime one.
Other extension accessors already have this kind of SFINAE restrictions.

PiperOrigin-RevId: 602418580

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

@@ -754,7 +754,8 @@ void MessageGenerator::GenerateFieldAccessorDeclarations(io::Printer* p) {
     // names.
     p->Emit(R"cc(
       template <typename _proto_TypeTraits, $pbi$::FieldType _field_type,
-                bool _is_packed>
+                bool _is_packed,
+                typename = typename _proto_TypeTraits::Singular>
       inline bool HasExtension(
           const $pbi$::ExtensionIdentifier<$Msg$, _proto_TypeTraits,
                                            _field_type, _is_packed>& id) const {
@@ -774,7 +775,8 @@ void MessageGenerator::GenerateFieldAccessorDeclarations(io::Printer* p) {
       }
 
       template <typename _proto_TypeTraits, $pbi$::FieldType _field_type,
-                bool _is_packed>
+                bool _is_packed,
+                typename = typename _proto_TypeTraits::Repeated>
       inline int ExtensionSize(
           const $pbi$::ExtensionIdentifier<$Msg$, _proto_TypeTraits,
                                            _field_type, _is_packed>& id) const {