if cond then
   return query select x,y, int
else
  return query select x,y, bool
end if;

When the return type of a PL/pgSQL function is declared as a polymorphic type (anyelement, anyarray, anynonarray, anyenum, or anyrange), a special parameter $0 is created. Its data type is the actual return type of the function, as deduced from the actual input types (see Section 35.2.5). This allows the function to access its actual return type as shown in Section 40.3.3. $0 is initialized to null and can be modified by the function, so it can be used to hold the return value if desired, though that is not required. $0 can also be given an alias. For example, this function works on any data type that has a + operator:

CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
RETURNS anyelement AS $$
DECLARE
    result ALIAS FOR $0;
BEGIN
    result := v1 + v2 + v3;
    RETURN result;
END;
$$ LANGUAGE plpgsql;

The same effect can be had by declaring one or more output parameters as polymorphic types. In this case the special $0 parameter is not used; the output parameters themselves serve the same purpose. For example:

CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
                                 OUT sum anyelement)
AS $$
BEGIN
    sum := v1 + v2 + v3;
END;
$$ LANGUAGE plpgsql;