The mysterious pg_proc.protrftypes

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)