BUG #17240: <timestamptz> at time zone ... ; wrong result

The following bug has been logged on the website:

Bug reference: 17240
Logged by: Marek Läll
Email address: marek(dot)lall(at)eesti(dot)ee
PostgreSQL version: 13.4
Operating system: Debian
Description:

Hello!

# SELECT version();
version

------------------------------------------------------------------------------------------------------------------------
PostgreSQL 13.4 (Debian 13.4-1.pgdg100+1) on aarch64-unknown-linux-gnu,
compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
(1 row)

# SET TimeZone='UTC';

# select * from pg_timezone_names where name like 'America/Los_Angeles';
name | abbrev | utc_offset | is_dst
---------------------+--------+------------+--------
America/Los_Angeles | PDT | -07:00:00 | t
(1 row)

# select now()
, now() at time zone 'America/Los_Angeles' as correct
, now() at time zone '-07:00:00' as wrong;

now | correct |
wrong
-------------------------------+----------------------------+----------------------------
2021-10-21 09:29:07.591962+00 | 2021-10-21 02:29:07.591962 | 2021-10-21
16:29:07.591962
(1 row)

--> COLUMN "WRONG" IS EXPECTED TO HAVE THE SAME VALUE AS COLUMN "CORRECT"

# select timestamptz'2021-10-01 00:00:00 UTC' at time zone
'America/Los_Angeles' as correct_1
, timestamptz'2021-10-01 00:00:00 UTC' at time zone '-07:00' as wrong_1
, timestamptz'2021-10-01 00:00:00 America/Los_Angeles' at time zone
'America/Los_Angeles' as correct_2
, timestamptz'2021-10-01 00:00:00 -07:00:00' at time zone '-07:00' as
wrong_2
, timestamptz'2021-10-01 00:00:00 -07:00:00' at time zone
'America/Los_Angeles' as correct_3
, timestamptz'2021-10-01 00:00:00 America/Los_Angeles' at time zone
'-07:00' as wrong_3;

correct_1 | wrong_1 | correct_2 |
wrong_2 | correct_3 | wrong_3
---------------------+---------------------+---------------------+---------------------+---------------------+---------------------
2021-09-30 17:00:00 | 2021-10-01 07:00:00 | 2021-10-01 00:00:00 |
2021-10-01 14:00:00 | 2021-10-01 00:00:00 | 2021-10-01 14:00:00
(1 row)

--> COLUMNS "WRONG_<N>" ARE EXPECTED TO HAVE THE SAME VALUE AS COLUMN
"CORRECT_<N>"