mutation fixes

pkg/cart/mutation_items.go

@@ -87,15 +87,25 @@ func (c *CartMutationContext) AddItem(g *CartGrain, m *cart_messages.AddItem) er
 		return fmt.Errorf("AddItem: invalid quantity %d", m.Quantity)
 	}
 
-	// Merge with any existing item having the same item id and matching StoreId
-	// (including both nil). Identity is the id; SKU is reference-only.
+	// Merge with any existing item having the same item id, matching
+	// StoreId, and matching ParentId (all three including both nil).
+	// Identity is ItemId; SKU is reference-only.
+	//
+	// ParentId MUST match: a standalone root line (ParentId == nil) and
+	// a child line (ParentId == &someParentLine) that happen to share an
+	// ItemId are different roles in the cart, even though the catalog
+	// item is the same. Merging them would silently convert an accessory
+	// into a +N on the root (or vice versa), corrupting the parent-child
+	// link. Both checks are delegated to equalOptional so the nullable-
+	// equality idiom stays in one place (cart-mutation-helper.go).
 	for _, existing := range g.Items {
 		if existing.ItemId != m.ItemId {
 			continue
 		}
-		sameStore := (existing.StoreId == nil && m.StoreId == nil) ||
-			(existing.StoreId != nil && m.StoreId != nil && *existing.StoreId == *m.StoreId)
-		if !sameStore {
+		if !equalOptional(existing.StoreId, m.StoreId) {
+			continue
+		}
+		if !equalOptional(existing.ParentId, m.ParentId) {
 			continue
 		}
 		if g.Type == cart_messages.CartType_REGULAR && c.UseReservations(existing) {