PathElement::validateMatchedPath

public static PathElement::validateMatchedPath(&$element, FormStateInterface $form_state, &$complete_form)

public static function validateMatchedPath(&$element, FormStateInterface $form_state, &$complete_form) {
  if (!empty($element['#value']) && ($element['#validate_path'] || $element['#convert_path'] != self::CONVERT_NONE)) {
    /** @var \Drupal\Core\Url $url */
    if ($url = \Drupal::service('path.validator')->getUrlIfValid($element['#value'])) {
      if ($url->isExternal()) {
        $form_state->setError($element, t('You cannot use an external URL, please enter a relative path.'));
        return;
      }
      if ($element['#convert_path'] == self::CONVERT_NONE) {
        // Url is valid, no conversion required.
        return;
      }
      // We do the value conversion here whilst the Url object is in scope
      // after validation has occurred.
      if ($element['#convert_path'] == self::CONVERT_ROUTE) {
        $form_state->setValueForElement($element, array(
          'route_name' => $url->getRouteName(),
          'route_parameters' => $url->getRouteParameters(),
        ));
        return;
      }
      elseif ($element['#convert_path'] == self::CONVERT_URL) {
        $form_state->setValueForElement($element, $url);
        return;
      }
    }
    $form_state->setError($element, t('This path does not exist or you do not have permission to link to %path.', array(
      '%path' => $element['#value'],
    )));
  }
}