LinkTitleRequiredConstraint.php

<?php

declare (strict_types=1);
namespace Drupal\link\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * Validation constraint for link title subfields if a URL was entered.
 */
class LinkTitleRequiredConstraint extends SymfonyConstraint {
  
  /**
   * The error message.
   *
   * @var string
   */
  public $message = "The Link text field is required if there is URL input.";

}