BUG #5975: Incorrect result from mod function with cast

The following bug has been logged online:

Bug reference:      5975
Logged by:          John Dickson
Email address:      jdickson@tnsi.com
PostgreSQL version: 8.3.11
Operating system:   RedHat EL 5.6
Description:        Incorrect result from mod function with cast
Details:

The mod function (and the % operator) can return an incorrect result when
combined with a cast().

The following SQL (executed on 8.3.11) shows a result one higher than the
maximum permissible remainder when combined with a cast:

-----
select 1129590 % 66,
  cast(1129590 as numeric(21, 0)) % 66;

 ?column? | ?column?
----------+----------
        0 |       66
(1 row)
-----

When executed on Postgres 8.2.5 (on Centos 4.6), the results are:
-----
 ?column? | ?column?
----------+----------
        0 |        0
(1 row)
-----

We're using the cast here to demonstrate the bug, but we actually strike the
problem when operating on values from a numeric(21, 0) column rather than
using cast().

We haven't tested this on later releases of 8.3, but didn't see any likely
fixes in the changelog.