interface DummyBackedEnumInterface extends \BackedEnum
{
}

    public static function enum(string $className, ?BuiltinType $backingType = null): EnumType
    {
        if (is_subclass_of($className, \BackedEnum::class)) {
            if (null === $backingType) {
                $reflectionBackingType = (new \ReflectionEnum($className))->getBackingType(); // <-- throws 
                $typeIdentifier = TypeIdentifier::INT->value === (string) $reflectionBackingType ? TypeIdentifier::INT : TypeIdentifier::STRING;
                $backingType = new BuiltinType($typeIdentifier);
            }

            return new BackedEnumType($className, $backingType);
        }

        return new EnumType($className);
    }