B-tree + sorting + unique constraint

Hi, I have a follow-up question to my earlier question[1] about how
sorting works in an index.

Does creating a UNIQUE constraint not allow the aforementioned sorting
capability?

eg.

create table t (x int, y int, z int);
insert into t values (1,1,1),(2,2,2),(3,3,3),(1,2,3),(15,23,21);

-- works:
create unique index t_uidx on t (x desc nulls last,y desc nulls last, z asc);
drop index t_uidx;

-- works:
alter table t add constraint t_uc unique (x,y,z);
alter table t drop constraint t_uc;

-- creating the unique constraint with sorting fails:
alter table t add constraint t_xyz_uc unique (x desc nulls last,y desc
nulls last, z asc);

ERROR: syntax error at or near "desc"
LINE 1: alter table t add constraint t_uidx unique (x desc nulls las...

[1] - http://archives.postgresql.org/pgsql-general/2010-12/msg00959.php