BUG #15042: Parition by list using enums odd behaviour

The following bug has been logged on the website:

Bug reference:      15042
Logged by:          Damir Ciganović-Janković
Email address:      damir.ciganovic.jankovic@gmail.com
PostgreSQL version: 10.1
Operating system:   Windows 10
Description:

First time asking question, if you like stackoverflow, check this link

https://stackoverflow.com/questions/48502345/postgresql-10-partition-by-list-using-enums?noredirect=1#comment84079025_48502345

If not, I will explain. I am trying to partition my table using enum as
partition condition, this is what I did:

create type positivity as enum (
    'POSITIVE',
    'NEGATIVE'
);

create table test (id int, polarity positivity) partition by list
(polarity);
create table test_1 partition of test for values in ('POSITIVE');
create table test_2 partition of test for values in ('NEGATIVE');

####

explain select * from test where polarity = 'NEGATIVE';
(output:)
 explain select * from test where polarity = 'NEGATIVE';
                          QUERY PLAN
--------------------------------------------------------------
 Append  (cost=0.00..76.50 rows=22 width=8)
   ->  Seq Scan on test_1  (cost=0.00..38.25 rows=11 width=8)
         Filter: (polarity = 'NEGATIVE'::positivity)
   ->  Seq Scan on test_2  (cost=0.00..38.25 rows=11 width=8)
         Filter: (polarity = 'NEGATIVE'::positivity)
(5 rows)


####
I expected that it will only scan test_2, adding constraints on both
partitions will get me desired behaviour:

alter table test_1 add constraint test_1_check check(polarity='POSITIVE');
alter table test_2 add constraint test_2_check check(polarity='NEGATIVE');

explain select * from test where polarity = 'NEGATIVE';
(output:)
                          QUERY PLAN
--------------------------------------------------------------
 Append  (cost=0.00..38.25 rows=11 width=8)
   ->  Seq Scan on test_2  (cost=0.00..38.25 rows=11 width=8)
         Filter: (polarity = 'NEGATIVE'::positivity)
(3 rows)

###

Is this a bug or is it not implemented yet maybe? Or is it working as
intended? 
Thank you in advance