**Symfony version(s) affected**: 5.1.7 ``` NOTICE: PHP message: PHP Fatal error: Uncaught Error: Call to undefined method ReflectionUnionType::isBuiltin() in <project>/vendor/symfony/dependency-injection/Dumper/Preloader.php:116 ``` **Description** <!-- A clear and concise description of the problem. --> **How to reproduce** Just trying to preload the framework generated preload file in cache dir.