ACP2E-2174: allow qty change for cart items without errors

Author: alzota

app/code/Magento/Checkout/Controller/Cart/UpdateItemQty.php

@@ -131,11 +131,9 @@ private function updateItemQuantity(Item $item, float $qty)
         if ($qty > 0) {
             $item->clearMessage();
             $item->setHasError(false);
-            $oldQty = $item->getQty();
             $item->setQty($qty);
 
             if ($item->getHasError()) {
-                $item->setQty($oldQty);
                 throw new LocalizedException(__($item->getMessage()));
             }
         }

app/code/Magento/Checkout/Model/Cart.php

@@ -19,6 +19,7 @@
  * @SuppressWarnings(PHPMD.CookieAndSessionMisuse)
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  * @SuppressWarnings(PHPMD.ExcessiveClassComplexity)
+ * @SuppressWarnings(PHPMD.TooManyPublicMethods)
  * @deprecated 100.1.0 Use \Magento\Quote\Model\Quote instead
  * @see \Magento\Quote\Api\Data\CartInterface
  * @since 100.0.2
@@ -535,6 +536,9 @@ public function updateItems($data)
 
             $qty = isset($itemInfo['qty']) ? (double)$itemInfo['qty'] : false;
             if ($qty > 0) {
+                if ($item->getHasError() && !$item->getProduct()->isInStock()) {
+                    $item->setUseOldQty(true);
+                }
                 $item->clearMessage();
                 $item->setHasError(false);
                 $item->setQty($qty);