update several columns from function returning several values

Hey,
a trivial question I guess,
can't make it work.

IF I define a function returning several values , I can't use it to update
mutliple columns of a table at once.
i __don't__ want to use CTE or subquerry,
and of course I don't wan tto compute the function several time.

CREATE TABLE test_update_m_values (
gid int,
gid_plus_1 int,
gid_minus_1 int
);

INSERT INTO test_update_m_values VALUES (1,0,0) ;

CREATE OR REPLACE FUNCTION rc_test_update_m_values( gid int,OUT gid_plus_1
int, OUT gid_minus_1 int)
AS $$ -- @brief : test function, can be deleted
BEGIN
SELECT gid+1,gid-1 INTO gid_plus_1, gid_minus_1;
RETURN ;
END;
$$ LANGUAGE 'plpgsql' IMMUTABLE STRICT ;

UPDATE test_update_m_values SET (gid_plus_1,gid_minus_1) =
(rc_test_update_m_values(gid)); --doesn't work

Somebody now if this is possible?

CHeers,
Rémi-C