private function tpl(): string
    {
        if (interface_exists($this->type)) {
            return 'Return value of %s must implement interface %s, %s returned';
        }

        if (class_exists($this->type)) {
            return 'Return value of %s must be an instance of %s, %s returned';
        }

        if ($this->type == 'callable') {
            return 'Return value of %s must be %s, %s returned';
        }

        return 'Return value of %s must be of the type %s, %s returned';
    }