polymorphic SQL functions has a problem with domains

Hello

I was informed about impossibility to use a polymorphic functions together
with domain types

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

is this bug?

Regards

Pavel Stehule