polymorphic SQL functions has a problem with domains

see

 create domain xx as numeric(15);

create or replace function g(anyelement, anyelement)
returns anyelement as
$$  select $1 + $2 $$
language sql immutable;

postgres=# select g(1::xx, 2::xx);
ERROR:  return type mismatch in function declared to return xx
DETAIL:  Actual return type is numeric.
CONTEXT:  SQL function "g" during inlining