BUG #4515: UPDATE strange behavior

The following bug has been logged online:

Bug reference: 4515
Logged by: xtron
Email address: xtronz(at)yandex(dot)ru
PostgreSQL version: 8.3.5
Operating system: Debian GNU/Linux
Description: UPDATE strange behavior
Details:

IMHO this is strange behavior:

create table a1(id int, value int);
create table a2(id int, value int);

this is work:
update a1 set value = a2.value from a2 where a1.id = a2.id;

this is NOT work:
update a1 set a1.value = a2.value from a2 where a1.id = a2.id;