It would be handy if portage could do operations on trees not currently in the overlay (perhaps they're development and not desired to accidentally be emerged, etc.). Example: ebuild foo.ebuild digest works in-PORTDIR{,_OVERLAY}, not out of it I couldn't find a duplicate bug, although there probably is one -- this seems to me like it would be a pretty common request.
*** This bug has been marked as a duplicate of 58677 ***