protected function createChildContext(array $parentContext, $attribute/*, string $format = null */)
    {
        if (\func_num_args() >= 3) {
            $format = \func_get_arg(2);
        } else {
            // will be deprecated in version 4
            $format = null;
        }

        $context = parent::createChildContext($parentContext, $attribute, $format);
        // format is already included in the cache_key of the parent.
        $context['cache_key'] = $this->getCacheKey($format, $context);

        return $context;
    }