BUG #7899: allow key word as alias in subquery but Can't reference it in outer query

The following bug has been logged on the website:

Bug reference: 7899
Logged by: jov
Email address: amutu(at)amutu(dot)com
PostgreSQL version: 9.1.3
Operating system: CentOS 6
Description:

xxx=# select 2 from (select 1 as end) t;
?column?
----------
2
(1 row)

xxx=# select end from (select 1 as end) t;
ERROR: syntax error at or near "end"
LINE 1: select end from (select 1 as end) t;
^

xxx=# select version();
version


------------------------------------------------------------------------------------------------------------
--
PostgreSQL 9.1.3 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.5
20110214 (Red Hat 4.4.5-6), 64-bi
t
(1 row)

I think use key word as alias should get a error message such as $key_word
is Key Word,should not be used as identifier bla bla,so for some complex
query error user can save a lot of time.