BUG #2003: Bug with SQL UPDATE on a NUMERIC

The following bug has been logged online:

Bug reference: 2003
Logged by: David Pujol
Email address: dpujol(at)toulouse(dot)inra(dot)fr
PostgreSQL version: 8.0 and 7.4
Operating system: Windows and linux
Description: Bug with SQL UPDATE on a NUMERIC
Details:

Hello, I've noticed a bug when I modify a quantity with an INSERT and a
minus operator.
My request is:
UPDATE products SET pr_stock=pr_stock--1 WHERE pr_code=600;

I haven't SQL error but 'pr_stock' value (numeric(5,1)) is unchanged : no
increment or decrement.

I've made tests:
- 'pr_stock=pr_stock +- 1' is OK (substraction -1)
- 'pr_stock=pr_stock ++ 1' is OK ( +1)
- 'pr_stock=pr_stock -+ 1' is OK ( -1)
- 'pr_stock=pr_stock -- 1' is not OK (no change, no error)

Same things with brackets :
- 'pr_stock=pr_stock +(- 1)' is OK (substraction -1)
- 'pr_stock=pr_stock +(+ 1)' is OK ( +1)
- 'pr_stock=pr_stock -(+ 1)' is OK ( -1)
- 'pr_stock=pr_stock -(- 1)' is not OK (no change, no error)

This bug report is for Windows XP + Postresql 8.0 and for Linux + Postgresql
7.4

PS : Sorry for my english ; I'm french!