BUG #13273: A query that returns wrongly labeled result instead of syntax error

The following bug has been logged on the website:

Bug reference:      13273
Logged by:          Missing comma in SELECT list results in wrong label without error
Email address:      v-brody@seznam.cz
PostgreSQL version: 9.4.1
Operating system:   CentOS Linux
Description:

Following query (notice the missing comma between top-level "x" and "y")
returns 1 (which is "x" value) under label "y" without any warning.

select
    "x" "y"
from
(
    select
        "a" as "x", "b" as "y"
    from
    (
        select 1 as "a", 2 as "b"
    ) "subquery1"
) "subquery2";

Expected behavior: syntax error

Environment:
PostgreSQL 9.4.1 on Linux version 2.6.32-504.16.2.el6.x86_64