ON CONFLICT with constraint name doesn't work

Hi,

I'm trying to explicitly use the constraint name of my UNIQUE INDEX but it
doesn't work (Postgres 9.6.2):

[local]:5432 nikolay(at)test=# create table constr(id serial, msg text);
CREATE TABLE

[local]:5432 nikolay(at)test=# create unique index i_constr_msg on constr
using btree(md5(msg));
CREATE INDEX

[local]:5432 nikolay(at)test=# insert into constr (msg) values ('hoho') on
conflict on constraint i_constr_msg do nothing;
ERROR: constraint "i_constr_msg" for table "constr" does not exist

[local]:5432 nikolay(at)test=# \d constr
Table "public.constr"
Column | Type | Modifiers
--------+---------+-----------------------------------------------------
id | integer | not null default nextval('constr_id_seq'::regclass)
msg | text |
Indexes:
"i_constr_msg" UNIQUE, btree (md5(msg))

This works:
[local]:5432 nikolay(at)test=# insert into constr (msg) values ('hoho') on
conflict (md5(msg)) do nothing;
INSERT 0 1

I don't see anything in the current docs
https://www.postgresql.org/docs/9.6/static/sql-insert.html saying that I
cannot use the unique index' name here. So it definitely looks like a bug.