domains missing some default castings

Hello

When I derive domain from some type I expect same castings rules.
Minimally for literals. But it's not true:

CREATE DOMAIN sixc AS char(6) NOT NULL;

CREATE OR REPLACE FUNCTION fxx(sixc)
RETURNS sixc AS $$
SELECT 'abcdef'; -- Actual return type is unknown.
$$ LANGUAGE SQL;
SELECT fxx('abcdef');

-- ok
CREATE OR REPLACE FUNCTION fxx(sixc)
RETURNS char(6) AS $$
SELECT 'abcdef';
$$ LANGUAGE SQL;

Regards
Pavel Stehule