aptitude upg<tab> gives:
And it is trying to tell me to choose 'full-upgrade' or 'safe-upgrade'. However, aptitude does have a 'upgrade' command.
aptitude - by the documentation - does not have an 'upgrade' command. But aptitude uses APT to do its job, and 'apt-get' does have an upgrade command.
Maybe the manual is missing this entry as well as the interface, and that is why the shell cannot expand that simple 'upgrade' option.
See this page: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=667578
Though it's an alias, I think that it won't cause too much trouble if the "aptitude" auto completion could support such alias.