Hi,

I'm thinking of converting a few ebuilds (x11-libs/qwt,
dev-libs/kqoauth, net-libs/qxmpp among them) to support building with
both Qt4 and Qt5.

Should this better be done by adding the corresponding useflags (qt4
and qt5 respectively) or by slotting? The pros and cons for each, off
the top of my head:

slotting:
+ Allows having different use flags for qt4 and qt5 builds (can't
think why that would be needed in the above examples though).
- Possibility of exponential growth of the number of slots in case
slotting would be required according to some other criteria (again,
can't think why that would be needed in the above examples).
- Requires keeping two different copies of the same ebuild with
basically the same build rules, with all the consequences.

useflags:
+ Seems to be easier and doing the required trick.
+ app-text/poppler already does this.
- Enabling support for previously disabled Qt version requires
rebuilding the whole library twice.

What's your opinion on this?

I've attached the useflag-based variant as a draft.

-- 
  Georg Rudoy