Bug report

Code snippet that reproduces the problem

/**
 * @param mixed $vcard
 * @return mixed
 */
public function enrichVcard($vcard)
{
    if (isset($vcard->FN)) {
        $vcard->FULLNAME = (string)$vcard->FN;
    }
}

gives

  Line   CardDav/Backend.php                                 
 ------ ---------------------------------------------------- 
  252    Access to an undefined property object::$FULLNAME.  
 ------ ---------------------------------------------------- 

Here is a similar stan: https://phpstan.org/r/2e641f19-1d8a-486c-8be6-06233e1ee9a8

Expected output

No error. It should be possible to intentionally create properties on objects.

Vcard is an actual Sabre\VObject\Document that uses getter/setter magic to provide dynamic properties. I need to convince stan that these properties exist. mixed doesn't work, object doesn't work, using the actual type as hint doesn't work, also another hint right above doesn't work:

    /** @var \stdClass $vcard */

I've re-read #928 but couldn't find the problem.