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@amutu.com
PostgreSQL version: 9.1.3
Operating system:   CentOS 6
Description:        =


xxx=3D# select 2 from (select 1 as end) t;   =

 ?column? =

----------
        2
(1 row)

xxx=3D# 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=3D# 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. =