Product can be added to cart even if the availability limit was reached

[Algorithman]

Describe the bug
A product with a quantity of 1 and with stock limitation can be put 2 times into the shopping cart.

To Reproduce
Steps to reproduce the behavior:
In the backend:
Create a product
Set 'Manage inventory method' to 'Track Inventory'
Set Stock Quantity to 1
Set 'low stock activity' to 'Unpublish'
Set Minimum cart quantity and Maximum cart quantity to 1
Set Backorders to 'No Backorders'

In the Frontend:
Add the product to the cart
Add the product to the cart again.

Expected behavior
The product shouldn't be added a second time.

Desktop (please complete the following information):

• OS: Windows 10 x64
• Browser chrome

Additional context
The problem lies in the call to the cart validator in ShoppingCartService.cs line 197.
There the new quantity isn't sent to the validator since the existingCartItem's quantity wasn't updated to the new value.

This should be fixed if we move

                // Update cart item
                existingCartItem.Quantity = newQuantity;

up a few lines so the validator gets the right values.

Also when you enable "Add products to cart in single positions" then you can add the product until you reach the cart item limit.
The validator seems to need some love :)

[Algorithman]

Is there a ETA on the 5.1.0 milestone?

[mgesing]

Thank you for reporting. A negative value for stock seems ok to me because that might be needed in special cases. There is no ETA for 5.1.0 yet.

[Algorithman]

Negative value for a limited quantity product also can be very unwelcome.
If you have only 1 left and can't reorder/produce again, you can't sell it twice.
There certainly are cases where it is ok, but also there are cases where it shouldn't happen at all.
If I set the inventory settings like in my image then I don't want any negative stock, that would just lead to unsatisfied/angry customers and confusion in the company why it could be oversold.

As for the bug itself, I didn't look into bundled items yet, but I think the same problem exists there.
The case with single positions in cart is easily solved with 5-10 lines of code (I did so now with my own validator) as well as the case with no single positions (also a easy fix), both cases for products with no attributes/bundles.
For the bundling and attributes I have no solution yet, but I don't need them yet either :)

[Algorithman]

And why did you close it, @mgesing ? The bug is still there.

[mgesing]

What bug do you mean? The bug described on top of this issue was fixed by 4f1070d.

[Algorithman]

Also the one when the cart is set to single items?
Same problem exists there and probably also for bundles.