BUG #17452: IN caluse behaves differently when there is one item comapred to when multiple

The following bug has been logged on the website:

Bug reference:      17452
Logged by:          Andreas Karlsson
Email address:      andreas@proxel.se
PostgreSQL version: 14.2
Operating system:   Linux and Mac at least
Description:

Hi,

I noticed that the optimization (in transformAExprIn()) for the case where
there is an IN (...) with only one item breaks certain queries. Of course it
is also possible that it is the single-item case which is correct and the
multiple-item case which is broken. The case where I found this behavior was
caused by the casting rules around the reg* types but there could be other
similar issues.

Here is a simplified example based on a real query:

# SELECT version();
                                                        version
                                           

------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 14.2 (Debian 14.2-1.pgdg120+1+b2) on x86_64-pc-linux-gnu,
compiled by gcc (Debian 11.2.0-18) 11.2.0, 64-bit
(1 row)

# SELECT 'version'::regproc IN ('version');
ERROR:  invalid input syntax for type oid: "version"
LINE 1: SELECT 'version'::regproc IN ('version');
                                      ^
# SELECT 'version'::regproc IN ('version', 'version');
 ?column? 
----------
 t
(1 row)

Best regards,
Andreas