Incorrect value of the reduced price in the cart

[GabiDivante]

Current behavior

Currently, then reduced price visible in the cart is not correctly calculated:
Reduced price = (net price * discount) + (net price * tax)

For example:
Net product price is 24, tax (23%) is 5.92 - so the full price of the product is 29.92.
The customer adds a 50% discount coupon.
Reduced price = (24 * 0,5) + (24 * 0,23) = 12 + 5,92 = 17,92

Despite this, all values ​​shown in the summary are correct, the client pays the correct amount.

Expected behavior

The tax value should also be reduced by the discount.
Reduced price = (net price * discount) + ((net price * discount) * tax)
So the tax after the reduction is 2.76, and the correct reduced price of the product is 14.76.

Steps to reproduce the issue

1. Go to https://demo.storefrontcloud.io/
2. Add any product to the cart
3. Apply discount code 'mmtest' (-50% discount)

Can you handle fixing this bug by yourself?

• YES
• NO

Which Release Cycle state this refers to? Info for developer.

Pick one option.

• This is a bug report for test version on https://test.storefrontcloud.io - In this case Developer should create branch from develop branch and create Pull Request 2. Feature / Improvement back to develop.
• This is a bug report for current Release Candidate version on https://next.storefrontcloud.io - In this case Developer should create branch from release branch and create Pull Request 3. Stabilisation fix back to release.
• This is a bug report for current Stable version on https://demo.storefrontcloud.io and should be placed in next stable version hotfix - In this case Developer should create branch from hotfix or master branch and create Pull Request 4. Hotfix back to hotfix.

Environment details

• Browser: Chrome 73
• OS: Ubuntu 18.04 LTS

Additional info

The same happens when any other discount rule is applied (any cart or catalog price rule).

[pkarw]

I'd suggest starting checking how Microcart/Product.vue displays the special prices - then go thru taxcalc.js to check if we don't have a flaw there; checking the Magento2 tax settings would be also recommended

[pkarw]

Fixed! :)

[GabiDivante]

@pkarw It works fine :) Though, I haven't thought about the order summary in the checkout, we have to change it the same way

By the way, take a look at my comments in #2874 - we have to solve the problem of displaying reduced prices (or correct value of discount) in instant checkout.