BUG #7498: Questionable interval parsing when in the form 'n m days'

The following bug has been logged on the website:

Bug reference: 7498
Logged by: Daniel Farina
Email address: daniel(at)heroku(dot)com
PostgreSQL version: 9.1.4
Operating system: Ubuntu 12.04
Description:

This is an expression that should probably have an error and not evaluate to
"true":

select '1 5 hours'::interval = '1 day 5 hours'::interval;
?column?
----------
t
(1 row)

I think that the first spelling, a unit-less '1', should not be accepted.
Other unit combinations agree:

select '1 5 minutes'::interval;
ERROR: invalid input syntax for type interval: "1 5 minutes"
LINE 1: select '1 5 minutes'::interval;
^
select '1 5 months'::interval;
ERROR: invalid input syntax for type interval: "1 5 months"
LINE 1: select '1 5 months'::interval;
^
select '1 5 seconds'::interval;
ERROR: invalid input syntax for type interval: "1 5 seconds"
LINE 1: select '1 5 seconds'::interval;