The AnnotationFileLoader interprets ClassName::class syntax as start of class definition and treats next token (if it's a string) as a class name.

Example:

trait Foo
{
    public function doBar()
    {
        $baz = self::class;  // this is parsed as a beginning of class definition
        if (true) {
        }
    }
}

AnnotationFileLoader will find "true" as a name of a class in that file.
This will result in "Class true does not exist" exception thrown from \Symfony\Component\Routing\Loader\AnnotationClassLoader::load.