code: public static <ContainingType extends MessageLite, Type>
        GeneratedExtension<ContainingType, Type>
            newSingularGeneratedExtension(
                final ContainingType containingTypeDefaultInstance,
                final Type defaultValue,
                final MessageLite messageDefaultInstance,
                final Internal.EnumLiteMap<?> enumTypeMap,
                final int number,
                final WireFormat.FieldType type) {
      return new GeneratedExtension<ContainingType, Type>(
          containingTypeDefaultInstance,
          defaultValue,
          messageDefaultInstance,
          new ExtensionDescriptor(enumTypeMap, number, type,
                                  false /* isRepeated */,
                                  false /* isPacked */));
    }
docs: For use by generated code only.