depended on table types

Is one supposed to be able to alter the type of a table whose definition
has been used A composite in another table? Somewhat surprisingly to
me, the following test did not produce an error:

create table a( x text, y int);
create table b( z a);
insert into b values('(\'aaa\',3)');
select * from b;
alter table a add column q timestamp not null;
select * from b;

cheers

andrew