Preload fails with error related to reflection in PHP8.0rc2 #38836
Description
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
How to reproduce
Just trying to preload the framework generated preload file in cache dir.