BUG #18151: pg_upgradecluster fails when column default refers to column

The following bug has been logged on the website:

Bug reference:      18151
Logged by:          Liam Morland
Email address:      liam@morland.ca
PostgreSQL version: 15.4
Operating system:   Debian
Description:

I was trying to upgrade to PG 16:
pg_upgradecluster 15 main

I got this error:
pg_restore: error: could not execute query: ERROR:  relation "event" does
not exist

The reason is the the default value for column "event_id" of table "event"
is "event_id_nextval()". That function is:
SELECT COALESCE(MAX(event_id) + 1, 1) FROM event;

I suppose that what is happening is a circular dependency, the table "event"
does not exist yet, because it is in the process of restoring it. I was able
to clear the error by setting the column default to NULL, running
pg_upgradecluster, and restoring the column default back to
"event_id_nextval()".

I did not have to do this on previous upgrades, so something has changed. In
the past, it just worked.

I do not understand why it would have to execute "event_id_nextval()" to do
the restore. All the rows already have a value in that column (they have to,
it is the primary key). (Yes, I am aware of SERIAL.)

A related issue: It would have been better if pg_upgradecluster had not
output a success message and switched to the new cluster. It should have
detected the error, output an error message, and left the old cluster
running.