Postgres 13

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE tbl (id int, txt text, val int);
INSERT INTO tbl VALUES
  (1, 'foo', 3)
, (2, 'bar', 4);

CREATE OR REPLACE FUNCTION change_val(_tbl_type anyelement)
  RETURNS SETOF anyelement  -- problem solved
  LANGUAGE plpgsql AS
$func$
BEGIN
   RETURN QUERY EXECUTE format(
      'UPDATE %s SET val = 2 RETURNING *;'
    , pg_typeof(_tbl_type)
     );
END
$func$;

CREATE TABLE

INSERT 0 2

CREATE FUNCTION

TABLE tbl;

id
txt
val

1
foo
3

2
bar
4

SELECT 2

SELECT * FROM change_val(NULL::tbl);

id
txt
val

1
foo
2

2
bar
2

SELECT 2

TABLE tbl;

id
txt
val

1
foo
2

2
bar
2

SELECT 2