Re: BUG #17479: "plan should not reference subplan's variable" when calling `grouping` on result of subquery

We may have more to do here, though, because with this patch I get

explain verbose
SELECT
    grouping(res.cnt)
FROM Card
CROSS JOIN LATERAL
(SELECT
    (SELECT Card.id) AS cnt
) AS res
GROUP BY
    res.cnt;

 HashAggregate  (cost=67.38..71.88 rows=200 width=8)
   Output: GROUPING((SubPlan 1)), ((SubPlan 2))
   Group Key: (SubPlan 2)
   ->  Seq Scan on public.card  (cost=0.00..61.00 rows=2550 width=8)
         Output: (SubPlan 2), card.id
         SubPlan 2
           ->  Result  (cost=0.00..0.01 rows=1 width=4)
                 Output: card.id

What became of SubPlan 1?  Maybe this is fine but it looks a little
shaky.  We should at least run down why that's happening and make
sure we're not leaving dangling pointers anywhere.