The mysterious pg_proc.protrftypes

Hi,

I'm trying to understand how pg_proc.protrftypes works.

The documentation says "Data type OIDs for which to apply transforms.".
For this column, there is no reference to any catalog table?
I would guess it should be "(references pg_type.oid)", right?

I tried to generate a value for this column to verify my hypothesis,
but I struggle to find an example that produces a not null value here.

I grepped the sources and found the "CREATE TRANSFORM FOR type_name" command,
and found an extension using it named "bool_plperl" which I installed.

I assumed this would cause a value, but no.

Both of bool_plperl's two functions get null pg_proc.protrftypes values.

I've tried running the full regression "make installcheck", but protrftypes doesn't seem to be covered:

$ cd postgresql
$ make installcheck
...
=======================
All 203 tests passed.
=======================
$ psql regression
regression=# SELECT COUNT(*) FROM pg_proc WHERE protrftypes IS NOT NULL;
count
-------
     0
(1 row)

Can someone please show me how to generate a function with a not null pg_proc.protrftypes value?

Many thanks.

/Joel