UserNameUnique.php

<?php

namespace Drupal\user\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
use Symfony\Component\Validator\Attribute\HasNamedArguments;

/**
 * Checks if a user name is unique on the site.
 */
class UserNameUnique extends UniqueFieldConstraint {
  public function __construct(mixed $options = NULL, ?bool $caseSensitive = NULL, $message = 'The username %value is already taken.', ?array $groups = NULL, mixed $payload = NULL) {
    parent::__construct($options, $caseSensitive, $message, $groups, $payload);
  }

}