Foreign key constraint on sub-column of composite-type column

Hi,

I was trying to create foreign key constraints on a sub-column of a composite-type column, but couldn't find a way to
doit. After asking around on IRC, it seems like this isn't supported in PostgreSQL. 

I wanted to do something like:
create type profile as (account_id integer);
create table users (profile profile);
alter table users add constraint account_fk foreign key ((profile).account_id) references accounts;

Would this be a viable feature request?

--David