Odd error message (" hash indexes do not support whole-index scans")

Shouldn't PostgreSQL behave in a different way in this case? (e.g. just
not use the index):

Table "public.forum_messages"

message_id | integer | not null default
nextval('forum_messages_message_id_seq'::text)
parent_userid | integer |
supersededby | integer | default 0

"forum_messages_pkey" primary key, btree (message_id)
"idx_supersede" hash (supersededby) WHERE (supersededby > 0)

# select m1.message_id,m2.message_id,m1.parent_userid,m2.parent_userid
from forum_messages m1, forum_messages m2 where
m2.supersededby=m1.message_id and m2.supersededby>0;
ERROR: hash indexes do not support whole-index scans

Regards,
Marinos