BUG #16586: deduplicate_items=true can be configured for numeric indexes

The following bug has been logged on the website:

Bug reference: 16586
Logged by: Matthias van de Meent
Email address: matthias(dot)vandemeent(at)cofano(dot)nl
PostgreSQL version: 13beta3
Operating system: Debian Stretch (9.13)
Description:

> CREATE INDEX numerical_index ON table USING btree ((num::numeric)) WITH
(deduplicate_items=true);
CREATE INDEX
> \d+ numerical_index
Index "public.numerical_index"
Column | Type | Key? | Definition | Storage | Stats target
--------+---------+------+------------+---------+--------------
num | numeric | yes | num | main |
btree, for table "public.table"
Options: deduplicate_items=true

There is no error for specifying the "deduplicate_items" -flag. As
deduplication is not supported for indexes with numeric type, I expected the
index creation statement to error.