protected function getVariantId(VariantInterface $variant = null)
 {
     return !$variant instanceof VariantInterface ? 0 : $variant->getId();
 }
 public function __construct(ProductInterface $product, VariantInterface $variant = null, $quantity, \Exception $previous)
 {
     $message = sprintf('Cannot add item with id: "%s", attribute: "%s" and quantity: "%s" to cart', $product->getId(), null === $variant ? 0 : $variant->getId(), $quantity);
     parent::__construct($message, $previous->getCode(), $previous);
 }