constraint exclusion with a tsrange type

[Ben Chobot <bench@xxxxxxxxxxxxxxx>]

Hey everybody, I'm having trouble getting constraint exclusion to work on a table partitioned with a tsrange type. I've distilled it down to this:

create table t (

  id serial primary key,

  observed_window tsrange not null

);

create index t_window on t(observed_window);

create table p1 (like t including all);

alter table p1 add check ( tsrange('2018-1-1','2019-2-1') @> observed_window);

alter table p1 inherit t;

create table p2 (like t including all);

alter table p2 inherit t;

alter table p2 add check ( tsrange('2018-2-1','2019-3-1') @> observed_window);

# explain select * from t where tsrange('2018-1-5','2018-1-6') @> observed_window;
                                          QUERY PLAN
-----------------------------------------------------------------------------------------------
 Append  (cost=0.00..51.75 rows=13 width=36)
   ->  Seq Scan on t  (cost=0.00..0.00 rows=1 width=36)
         Filter: ('["2018-01-05 00:00:00","2018-01-06 00:00:00")'::tsrange @> observed_window)
   ->  Seq Scan on p1  (cost=0.00..25.88 rows=6 width=36)
         Filter: ('["2018-01-05 00:00:00","2018-01-06 00:00:00")'::tsrange @> observed_window)
   ->  Seq Scan on p2  (cost=0.00..25.88 rows=6 width=36)
         Filter: ('["2018-01-05 00:00:00","2018-01-06 00:00:00")'::tsrange @> observed_window)
(7 rows)

I would have expected that postgres could reason that, because I'm asking for an observed_window that fits within the tsrange (2018-1-5,2018-1-6), no matter what it was, it would not be found in p2. Obviously postgres says I'm wrong, but I don't know why? 

Constraint exclusion is set to "partition".