Domains and function

I can create a function with a domain and
define it to return a domain.

The parameter is checked to see if it qualifies
in the constraint of the domain, however, the
return value is not.

Is this a bug?  Is the author of the function
responsible for re-inforcing the constraint
at runtime?

This is the test case in 7.4:

=# create domain one2hundred AS integer
-#    DEFAULT '1' CONSTRAINT email_domain check( VALUE > 0 AND VALUE <=100 );
CREATE DOMAIN
=#
=# create function gb52_add( one2hundred )
-# returns one2hundred as
-# '
'# BEGIN
'#    RETURN $1 + 10;
'# END;
'# ' language 'plpgsql';
CREATE FUNCTION
=#
=# select gb52_add( 80);
 gb52_add
----------
       90
(1 row)

=# select gb52_add( 100);
 gb52_add
----------
      110
(1 row)

=# select gb52_add( 90);
 gb52_add
----------
      100
(1 row)

=# select gb52_add( 91);
 gb52_add
----------
      101
(1 row)

=# select gb52_add( 191);
ERROR:  value for domain one2hundred violates check constraint "email_domain"


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