[Magento 2.1.2] Wrong error message in cart after entering invalid discount code with product stock qty = 1 (only for simple variant products)

[LucScu]

Tested on last magento 2 version 2.1.2 and Luma sample data.

Preconditions

1. Set a simple variant product qty to 1.
   
   With sample data i used
   
   http://luma.kamiceria.com/tiffany-fitness-tee.html
   
   with blue color and xs size (sku WS09-XS-Blue)

Steps to reproduce

1. Add above product WS09-XS-Blue to cart
2. Go to cart
3. Add invalid discount code like 'xxx'

Expected result

1. Magento shows error message
   
   'The coupon code "xxx" is not valid.'

Actual result

1. Magento shows error messages
   
   'The` coupon code "xxx" is not valid.' ->OK
   
   'We don't have as many "Tiffany Fitness Tee" as you requested.' ->BUG

[olysenko]

Hi, thank you for your reporting. Internal ticket is created MAGETWO-60352

[LucScu]

Ok the bug is reported, but when you fix it?

[olysenko]

Unfortunately, we don't have time estimates for this bugfix now

[LucScu]

Thank you, i hope you find the time to fix it asap.

[LucScu]

Do you know if this bug is fixed on 2.1.3?

[thelettuce]

Can confirm the problem still happens in 2.1.3.

[thelettuce]

It seems to be if there is not 2 * the number of items in your cart in stock, then the error happens.
eg.
if the item qty in-stock is 6 then
if you have 3 items in your cart and enter invalid discount there's no error.
if you have 4 items in your cart and enter invalid discount you get the error.

probably to do with the discount function copying the cart or something.