Does the "GPL version choice" impact GPL-compatibility?

[Glenn Maynard <glenn@zewt.org>]

On Wed, Aug 25, 2004 at 04:40:34PM -0400, Brian Thomas Sniffen wrote:
> And my changes do specify a version number.

Hmm. It comes down to compatibility again.  This isn't immediately
clear to me either way from reading the license; instead of trying
to interpret, I'm going to punt the question.

licensing@gnu.org, a question (in a few forms):

Is it valid to combine GPL work placed under "GPL version 2" with one
under "GPL version 2 or any later version"?  That is, do versioning
choices impact compatibility (when the versions overlap)?  Are all future
modifications bound to give the same permission to upgrade the GPL?

It seems like an added restriction; "version 2" implies "no upgrades
allowed".  It would allow a third party to prevent his modifications
from being used in the original work, if the original authors want to
maintain "or any later version", by not granting that permission in his
modifications (which is exactly something GPL#6 seeks to prevent).

If not, it's worse: GPL software, under a canonical interpretation, would
be incompatible with other GPL software due to differently exercised
license options.  As many people do, in fact, omit "or any later version",
it would probably be a real-world problem.

-- 
Glenn Maynard