Description

Caused by #13185, \Magento\Tax\Model\Sales\Total\Quote\Tax::clearValues() is called to unset the total before collecting total. null is expected (representing unset state) in below condition however 0 is set and failed the condition necessary to calculate correct shipping discount.

- expected
+ actual
// \Magento\SalesRule\Model\Validator::processShippingAmount
    public function processShippingAmount(Address $address)
    {
-        // $shippingAmount = null
+        // $shippingAmount = 0
        $shippingAmount = $address->getShippingAmountForDiscount();
        if ($shippingAmount !== null) {
            $baseShippingAmount = $address->getBaseShippingAmountForDiscount();
        } else {
            $shippingAmount = $address->getShippingAmount();
            $baseShippingAmount = $address->getBaseShippingAmount();
        }
        // ...
        $discountAmount = ($shippingAmount - $address->getShippingDiscountAmount()) * $rulePercent / 100;
        // ...
    }

Shipping discount fail to apply to an order, when:

• order is checking out with payment method which do not collect total after savePaymentInformation() (for example, credit card methods usually meet this criteria, while paypal methods not)
• order is applied with a promotional rule with shipping discount

Fixed Issues (if relevant)

Can't found any related issue so far

Manual testing scenarios

1. Create coupon of 20%off discount and set it to applicable to shipping
2. Apply the coupon
3. Check out with Braintree
4. After placed the order, go to account page to view the order

Expected result: Order discount contains shipping amount

Actual result: Order discount does not contain shipping amount

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)