GPLv2 to v3 upgrade and "further restrictions" issue

[Bradley M. Kuhn]

While IANAL, I do believe Janez Pers gives very good advice on this
thread.  As one of GPLv3's drafters, I can tell you that the reason this
part of GPLv3 was added:

     If the Program as you received it, or any part of it, contains a
     notice stating that it is governed by this License along with a
     term that is a further restriction, you may remove that term.

was precisely because many copyright holders over the years (sometimes
by accident and sometimes for nefarious purposes) have added
GPLv2-contradictory terms outside in a separate, accompanying license
notice.  (For example, adding things like: "No commercial distribution
permitted" right after a GPLv2 notice).  These types of contradictory
licenses have always been a problem in the Free Software community.
With GPLv3, you can just throw them away, but you can't take such
liberties under GPLv2.

Meanwhile, the GPLv2-or-later interaction here is a complicated
question, and before you try to exercise any of your
copyright-controlled freedoms with regard to the codebase in question
with the dubious extra restriction, you should consult legal counsel.

If you can't and/or don't want to talk to a lawyer, then simply
pretending the "or-later" isn't there and defaulting to the same
conservative stance you would take with GPLv2-only plus such a
restriction (i.e., that you have *no idea* what your license is and what
you're permitted to do) is probably the best you can do.
-- 

   -- bkuhn