BUG #17030: ERROR: cannot decompile join alias var in plan tree introduced in pg14beta1

The following bug has been logged on the website:

Bug reference:      17030
Logged by:          Hans Buschmann
Email address:      buschmann@nidsa.net
PostgreSQL version: 14beta1
Operating system:   Windows Server 2019 64 bit
Description:

On Windows I installed the new pg14beta1 downloaded from EDB Website.
xxxdb=# select version ();
                            version
---------------------------------------------------------------
 PostgreSQL 14beta1, compiled by Visual C++ build 1914, 64-bit
(1 Zeile)

When testing our application I suddenly got the $subject error message:

ERROR:  cannot decompile join alias var in plan tree

This occurs on a quite complex CTE query of the style (here only a
simplified layout to give an idea): 

with
qxxx as (
select a,b,c
from taba
left join tabb
...)
select xmlelement (name sel,
xmlelement(name bbb
...
))
from (select * from qxxx order by colorder) as qxo
)

This particular query (generating an XML-tree) worked for about 5 years on
all newest minor versions from 9.6 to 13.3.

I know that this is not an reproduceble case, but I can supply the execution
plan from pg13.3 if needed.

Thanks for investigation

Hans Buschmann