Tax rate + 100% discount results in negative grand total

[dankocherga]

Setting up tax a rule and applying 100% discount in the shopping cart results in a negative grand total.

Preconditions

1. Magento 2.1.7

Steps to reproduce

Admin setup

1. Create product A with price = 2 762,71
2. Create product B with price = 2 542,37
3. Setup tax rule 18%
4. Create coupon code that adds 100% discount.

Front-End

1. Add both products to the cart with qty = 2
2. Apply coupon code

Expected result

1. Grand total (without shipping yet calculated) = 0

Actual result

1. Grand total is -0,01

Calculating by hand

Product	Qty	Price	Row total	Row Tax
A	2	2762,71	5525,42	994,5756
B	2	2542,37	5084,74	915,2532

Total	Expected Amount
subtotal	10610,16
tax	1909,8288

How Magento calculates it
Putting a debugger at Magento\Quote\Model\Quote\Address\Total\Grand we could see:

Tax amount is rounded and it makes the total sum to be 10610,16 + 1909,83 - 12520 = -0,01

[Andrii-Deineha]

Hi @dankocherga, Thanks for reporting this issue. Unfortunately, I could not reproduce the issue as you described it.

[magento-engcom-team]

@dankocherga Please provide the detailed steps we must follow to reproduce this issue. Screenshots or logs would be helpful, too.

[dankocherga]

@magento-engcom-team
I have created a Magento database back-up with products and tax rates prepared: https://www.dropbox.com/s/i5kxzqg671zb9nh/1505994954_db_demo.sql.sql.zip?dl=0

Screenshot of the issue in the cart:

Screenshots of my tax settings:

[okorshenko]

@dankocherga thank you for update

[magento-engcom-team]

@dankocherga, thank you for your report.
We've created internal ticket(s) MAGETWO-77433 to track progress on the issue.