Unique constraints and indexes.

Is this a bug? I create a "unique" index, directly but it doesn't add a unique
constraint. Add a unique constraint and it adds the index and the constraint.
(pg version 9.4.5 on fedora 22, but also occurs in other versions).
Functionally I can't see a difference.

mydb=# create table test_table ( f1 bigint, f2 bigint);
CREATE TABLE
mydb=# create unique index test_table_un on test_table (f1, f2);
CREATE INDEX
mydb=# \d test_table
  Table "public.test_table"
 Column |  Type  | Modifiers
--------+--------+-----------
 f1     | bigint |
 f2     | bigint |
Indexes:
    "test_table_un" UNIQUE, btree (f1, f2)

mydb=# select conindid, contype, conname from pg_constraint where conname like
'test_table%';
 conindid | contype | conname
----------+---------+---------
(0 rows)



-- --------------------------------------------------------------------------
mydb=# drop table test_table;
DROP TABLE
mydb=# create table test_table ( f1 bigint, f2 bigint);
CREATE TABLE
mydb=# alter table test_table add constraint test_table_un unique (f1,f2);
ALTER TABLE
mydb=# \d test_table
  Table "public.test_table"
 Column |  Type  | Modifiers
--------+--------+-----------
 f1     | bigint |
 f2     | bigint |
Indexes:
    "test_table_un" UNIQUE CONSTRAINT, btree (f1, f2)

mydb=# select conindid, contype, conname from pg_constraint where conname like
'test_table%';
 conindid | contype |    conname
----------+---------+---------------
  4284073 | u       | test_table_un
(1 row)

mydb=#