pg_upgrade changes can it use CREATE EXTENSION?

I'm not too familiar with the innards of pg_upgrade, but we've been
discussing it a lot for past couple of days and how it's causing issues for
PostGIS upgrades.

I think this thread covers most of the issues.

https://lists.osgeo.org/pipermail/postgis-devel/2017-August/026355.html

My thought was is it possible for pg_upgrade to be taught to use CREATE
EXENSION if asked?

Right now we don't support PostgreSQL 11 on PostGIS 2.3 and we really would
like not to because there are too many changes done in 11 that we feel
queezy about backporting.
Even if we did, package maintainers would have to provide 2.3 on 11 and 2.4
on 11 just so people can pg_upgrade to PostgreSQL 11 and then

ALTER EXTESNION postgis UPDATE;

To postgis 2.4.0

Given that latest PostgreSQL 11 head already doesn't compile against PostGIS
2.4, I'm not confident we can fix 2.4 for 11. So this will continue to be
more of a problem especially at the rate that PostgreSQL is changing these
days.

Right now crafty users have to do something like this to use pg_upgrade

https://gist.github.com/Komzpa/994d5aaf340067ccec0e

My solution of let's not call it postgis-2.4 but just postgis-2 from
thenceforward for the life of 2 major series because we don't break backward
compatibility often in a PostGIS minor version got shot down.

Any thoughts on this?

Thanks,
Regina Obe
PostGIS PSC member