Mike Linksvayer wrote on 9 August:
there are cases where unambiguously MPL code could be included in a
copyleft-next project, if only copyleft-next allowed for it.
Honestly, I'm not convinced that a truly strong copyleft can allow
unilaterally inclusion of other copylefted code -- even weak -- without
bilateral compatibility clauses. We have to consider that such
provisions while seemly safe on the surface may actually be manipulated
to undermine the stronger copyleft by enterprising defense attorneys.
Richard Fontana wrote on 2 September:
This proposal might make the most sense for licenses that are
generally considered non-copyleft but GPL-incompatible (the proposed
language seems modeled on things like the [I believe FSF-recommended]
OpenSSL GPL linking exception). For copyleft licenses, it raises the
same problem on a more general level that we dealt with when thinking
about a clause that would achieve EPL compatibility.
Exactly. I think anyone who wants this kind of compatibility has
to prove that it can't be used by the nefarious to circumenvent copyleft.
Note that the ways copyleft is subverted are subtle and complicated. Most
people don't see that because they rarely get the rather horrible experience
of regularly dealing with people who are trying. You may have to take
my word for it that this happens, but if folks want to raise specific
questions, I'm happy to talk about it in detail. There's enough of it
and it's common enough that it's difficult to generalize.