Regression related to InvalidArgumentException

I think validation condition is overcomplicated:

      // Check the path validity
      // and whether the anonymous user can access the entered path.
      if (!UrlHelper::isExternal($r4032loginUserLoginPath)
        && (($r4032loginUserLoginPath != '<front>') || ($r4032loginUserLoginPath = Url::fromRoute($r4032loginUserLoginPath)->toString()))
        && (!$this->pathValidator->getUrlIfValidWithoutAccessCheck($r4032loginUserLoginPath)
          || !($url = Url::fromUserInput($r4032loginUserLoginPath))
          || !$url->access(User::getAnonymousUser()))
      ) {

The mentioned exception is throw by UrlHelper::isExternal, to check if url is external we could use:

$url = $this->pathValidator->getUrlIfValidWithoutAccessCheck($r4032loginUserLoginPath);
$url->isExternal();

I've created fork and merge request to 2.x which simplify this condition and update form element description which is outdated after this change. Updated test as well, as it was failing.

Although this fixes the exception, it allows for the path to be entered without the leading slash, which results in a fatal error when the user is redirected.

Also, the steps to reproduce as written requires the path saml_login to exist. The issue can be reproduced by simply removing the leading slash from the default value, /user/login path, and submitting the form.

ad0z → changed the visibility of the branch 3396702-regression-related-to to hidden.

Updated MR to add leading zero for internal urls if not available, added mentioned scenario to tests cases as well.