Fix negative credit memo #19899

[wojtekn]

Description

This fixes negative credit memo problem described in #19899. The problem looks as introduced in \Magento\Sales\Model\Order\Creditmemo\Total\Discount which calculates base_shipping_amount based on tax related variables if base_shipping_amount is zero:

private function getBaseShippingAmount(\Magento\Sales\Model\Order\Creditmemo $creditmemo): float
{
    $baseShippingAmount = (float)$creditmemo->getBaseShippingAmount();

    if (!$baseShippingAmount) {
        $baseShippingInclTax = (float)$creditmemo->getBaseShippingInclTax();
        $baseShippingTaxAmount = (float)$creditmemo->getBaseShippingTaxAmount();
        $baseShippingAmount = $baseShippingInclTax - $baseShippingTaxAmount;
    }

    return $baseShippingAmount;
}

I fixed this by setting base_shipping_incl_tax to 0 in \Magento\Sales\Model\Order\CreditmemoFactory::initData too:

$creditmemo->setBaseShippingInclTax((double)$data['shipping_amount']);

Fixed Issues (if relevant)

1. Credit memo for $0 order without refunded shipping produces negative credit memo #19899

Manual testing scenarios

1. Create Cart Price Rule with 100% discount, applied to shipping, available for everyone
2. Create simple product with positive stock and price = $100
3. Place order in backend for the created product with qty =1 and flat rate shipping ($5 by default).
4. Invoice order (but don't create shipment)
5. Navigate to Credit Memo form
6. Change Refund Shipping from 5 to 0
7. Click "Refund Offline"

Expected behavior is to have $0 credit memo created. Instead it creates credit memo with negative, -$5.0 total.

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)

[magento-engcom-team]

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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3720 has been created to process this Pull Request

[ghost]

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[magento-engcom-team]

Hi @wojtekn. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[magento-engcom-team]

Hi @wojtekn. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.