WC_Order_Item_Coupon::offsetSet PHP Method

    public function offsetSet($offset, $value)
    {
        if ('discount_amount' === $offset) {
            $offset = 'discount';
        } elseif ('discount_amount_tax' === $offset) {
            $offset = 'discount_tax';
        }
        parent::offsetSet($offset, $value);
    }