Sorry for bumping this discussion thread this late, but I discovered the Berkeley DB license* ("the BDB") and it made me think about what had been said here.
https://www.gnu.org/licenses/license-list.html#BerkeleyDB
Haven't looked at the BDB before, and I find it interesting. Taking into account only the numbered points, it might be the shortest example of a copyleft license in use in the field, even close to the shortest possible. It's close in form to a BSD-like license plus source requirement.
The FSF claims that the BDB is GPL compatible, I assume because it does not specify anything outside the bare minimum, in particular does not restrict any "further restrictions". Does it even propagate a guarantee of the four freedoms to modified versions of the software?
On 2012-10-12 13:45, Bradley M. Kuhn wrote:
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.
Is the BDB a counter-example to your thesis? Or is it not "truly strong"?
It seems to me that any license, that accepts whatever added conditions another license contains, will be forward-compatible with the more restrictive license. Either by enumerating certain allowed further restrictions, like the GPLv3 vs the AGPLv3, or by not explicitly forbidding any, like the BDB.
The point about using a weaker license to undermine a stronger one stands, whether if it means using a weaker, forward-compatible license to gradually rewrite the code base, or if it means some currently unknown license jujutsu.