#[\ReturnTypeWillChange] is an attribute added in PHP 8.1, which "signals that a mismatching tentative return type should not emit a deprecation notice". In my local environment, this is not throwing any errors in PHPCS, but CodeClimate reports it as an error:

The actual attribute can't be seen there, but the code being checked looks like this:

  /**
   * {@inheritDoc}
   */
  #[\ReturnTypeWillChange]
  public function jsonSerialize() {