Is this a Postgres Bug?

First off, I've posted this question on StackOverflow in case anyone
wants to answer it:


http://stackoverflow.com/questions/11814132/postgresql-smallint-overflowing-when-creating-index-on-multiple-columns-is-th

The repro can be found here: http://sqlfiddle.com/#!1/734d7/1

I'm happy to log this as a bug, unless someone can explain to me why
this behavior is by design.  Thanks!

Mike