Re: BUG #17709: Regression in PG15 with window functions - "WindowFunc not found in subplan target lists"

The additional thing that seems to cause the reported error is that
once the subquery is pulled up, the run condition also needs a round
of constant folding done. See subquery_planner() around line 827.  The
problem is that the target list's WindowFunc ends up with count(1)
over .., but the run condition's one is left as count(case 1 when 1
then 1 else null end), which preprocess_expression() will fold into
the same as what's in the target list.

I'm now wondering if WindowClause.runCondition should be of type Node
* instead of List *. I'd have imagined I should be passing the type of
EXPRKIND_QUAL to preprocess_expression's type, but canonicalize_qual()
does not like Lists.