Thank you Stephen Mack !

As a workaround we can use:

1) If you can insert output in last iteration then use like:

                <t t-set="total" t-value="0" />
                <t t-foreach="o.order_line" t-as="line">
                    <t t-set="total" t-value="total+line.quantity" />
                    <t t-if="line_last">
                        <t t-esc="total" />
                    </t>
                </t>

​​2) If you can't insert output in loop then use like:

                <t t-set="total" t-value="sum([line.quantity for line in o.order_line])" />