BUG #18348: Inconsistency with EXTRACT([field] from INTERVAL);

The following bug has been logged on the website:

Bug reference:      18348
Logged by:          Michael Bondarenko
Email address:      work.michael.2956@gmail.com
PostgreSQL version: 14.10
Operating system:   macOS
Description:        

Hello,

I'm building a random semantically-correct SQL code generator for PostgreSQL
and I stumbled upon an inconsistency:

tpch=# select extract(year from interval '3 years');
 extract 
---------
       3
(1 row)

tpch=# select extract(week from interval '3 weeks');
ERROR:  interval units "week" not supported

In the documentation it's mentioned that 'week' is an ISO 8601 week, so it
makes sense why it's not applicable to INTERVAL, which is the same for
isoyear. However, the field is named week and not isoweek, so I expect it to
work like the `select extract(year from interval '3 years');` does.
Moreover, the documentation does not mention that the field cannot be
extracted from INTERVAL, like it does for isoyear:
https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT
.