composer create-project symfony/website-skeleton preload "5.1.x-dev"
cat preload/var/cache/dev/App_KernelDevDebugContainer.preload.php | grep "bool\|string\|int"

$ cat test/var/cache/dev/App_KernelDevDebugContainer.preload.php | grep "bool\|string\|int"
$classes[] = 'int';
$classes[] = 'string';
$classes[] = 'Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntityValidator';
$classes[] = 'Symfony\Component\Security\Http\EntryPoint\RetryAuthenticationEntryPoint';
$classes[] = 'Symfony\Component\Security\Core\Validator\Constraints\UserPasswordValidator';
$classes[] = 'Symfony\Component\Serializer\Normalizer\ConstraintViolationListNormalizer';
$classes[] = 'bool';
$classes[] = 'Symfony\Component\Validator\ContainerConstraintValidatorFactory';
$classes[] = 'Symfony\Component\Validator\Constraints\EmailValidator';
$classes[] = 'Symfony\Component\Validator\Constraints\ExpressionValidator';
$classes[] = 'Symfony\Component\Validator\Constraints\NotCompromisedPasswordValidator';
$classes[] = 'Symfony\\Component\\Form\\Extension\\Validator\\Constraints\\Form';