BUG #18097: Immutable expression not allowed in generated at

The following bug has been logged on the website:

Bug reference:      18097
Logged by:          Jim Keener
Email address:      jim@jimkeener.com
PostgreSQL version: 15.0
Operating system:   Linux
Description:

Given this table:

CREATE TABLE test_table (
id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
created_at timestamptz NOT NULL DEFAULT now()
);

The following work:

* alter table test_table add created_local_y text GENERATED ALWAYS AS
(EXTRACT(isoyear FROM created_at AT TIME ZONE 'America/New_York')) STORED;

* alter table test_table add created_local_w text GENERATED ALWAYS AS
(EXTRACT(week FROM created_at AT TIME ZONE 'America/New_York')) STORED;

* alter table test_table add created_local text GENERATED ALWAYS AS
(EXTRACT(isoyear FROM created_at AT TIME ZONE 'America/New_York')::text ||
'|' || EXTRACT(week FROM created_at AT TIME ZONE 'America/New_York')::text)
STORED;

* CREATE INDEX ON test_table ((EXTRACT(isoyear FROM created_at AT TIME ZONE
'America/New_York') || '|' || EXTRACT(week FROM created_at AT TIME ZONE
'America/New_York')));

However, the following DOES NOT work with an error of (ERROR:  generation
expression is not immutable):

* alter table test_table add created_local text GENERATED ALWAYS AS
(EXTRACT(isoyear FROM created_at AT TIME ZONE 'America/New_York') || '|' ||
EXTRACT(week FROM created_at AT TIME ZONE 'America/New_York')) STORED;

Given that casting shouldn't "increase" the immutability of an expression,
and expression indexes need also be immutable afaik, I think that there is a
bug somewhere here?

Thank you,
Jim