mergeCart example

string $token,
        string $customerId
    ): SalesChannelContext {
        if (!$customerContext->getDomainId()) {
            $customerContext->setDomainId($currentContext->getDomainId());
        }

        $guestCart = $this->cartService->getCart($token, $currentContext);
        $customerCart = $this->cartService->getCart($customerContext->getToken(), $customerContext);

        if ($guestCart->getLineItems()->count() > 0) {
            $restoredCart = $this->mergeCart($customerCart, $guestCart, $customerContext);
        } else {
            $restoredCart = $this->cartService->recalculate($customerCart, $customerContext);
        }

        $restoredCart->addErrors(...array_values($guestCart->getErrors()->getPersistent()->getElements()));

        $this->deleteGuestContext($currentContext, $customerId);

        $errors = $restoredCart->getErrors();
        $result = $this->cartRuleLoader->loadByToken($customerContext, $restoredCart->getToken());