ForumLeafConstraint.php

<?php

namespace Drupal\forum\Plugin\Validation\Constraint;

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

/**
 * Checks that the node is assigned only a "leaf" term in the forum taxonomy.
 */
class ForumLeafConstraint extends SymfonyConstraint {
    public $selectForum = 'Select a forum.';
    public $noLeafMessage = 'The item %forum is a forum container, not a forum. Select one of the forums below instead.';

}