BUG #6202: type of union column selection bug

The following bug has been logged online:

Bug reference:      6202
Logged by:          Roman
Email address:      zotov@oe-it.ru
PostgreSQL version: 9.0.4
Operating system:   Win7x64
Description:        type of union column selection bug
Details:

Query
select null union select 4
is Ok but
select null union select null union select 4
is Fail

i think it`s a bug because you choose type in second query twice
null and null -> text
text and int -> fail

and i think what check type category is not true, because i can`t create
implicit cast to resolve some problems, like
select 'a' union select 4
if i have implicit cast int->text i think it must work!!

Sorry second part is not bug it`s a feature but it`s my problem too