USE=multislot, xDEPEND-syntax in SLOT and Slot dependencies [Was: Re: [RFC] some global useflags]

I'm deliberately starting a new thread here, because this is meandering
well off the original topic of the global useflag proposals.

On Thu, Oct 16, 2008 at 02:18:16AM +0200, Marius Mauch wrote:
> There are also other issues, e.g. it breaks the generation of the
> @installed package set as the installed slots can't be found in the
> tree. As portage-2.2 makes increased use of slot atoms internally for
> vdb handling we got a few bugreports simply due to the cache constraint
> violation by USE=multislot.
Ok, I found bug #174184 on this matter.

kugelfang proposed to include this in EAPI=1, but I don't find it in
there, what happened?

The short-term fix of use.mask just stops current breakage, but we need
a real solution so that we can keep using it where we actually need it.

Ignoring Vapier's tirade against ciaranm there, we need the
xDEPEND-syntax for SLOTS as the real solution, however that still
wouldn't resolve the portion that has CTARGET as part of the SLOT, since
metadata generated on the rsyncmaster with a different CTARGET wouldn't
match on the clients.

I'm also wondering about a bad interaction with slot dependencies even
without having CTARGET:
Say gcc has two variants, SLOT=3.4 and SLOT=3.4.5. 
The language in the approved version of PMS (section 9.2.5) states: "A
specification with a named slot dependency matches only if the slot of
the matched package is squal to ths lot specified."