> > +----------------------+
> > | allowed |
> > |+---------+ +-------+| +-------+
> > || Eclipse | | darcs || | Linux |
> > || CPL | | ? || | GPL |
> > |+---------+ +-------+| +-------+
> > +----------------------+
>
> Yes, but this case can't happen, once GPL code is part of darcs. Any
> code linking to darcs in any meaningful way inextricably links to GPL
> code we incorporate, reverting to the case above.
>
> So, I repeat, we don't have the ability to both strongly take advantage
> of other GPL code in darcs, and make darcs compatible with CPL licensed
> code.
You are right. If someday there is some GPL'ed code that could be used to
improve darcs I would fully approve of using it. If someone wanted to use
darcs in a CPL'ed project after that then they would be left with the option
of replacing or removing the GPL'ed code or getting a special exception from
the copyright holders of that code.
Likewise, including CPL'ed source code into darcs would render darcs
GPL-incompatible, and I would strongly advise against doing that.
However, adding the special exception for CPL clause to the darcs license does
not render darcs GPL-incompatible.
For example, suppose that someday IBM invents a fantastically wonderful
tool which they created by mixing together darcs and Eclipse source code, and
they distribute this tool to the world. The license that they would have to
use would say "X, Y and Z components of this software are under the CPL, and
the rest are under GPL.". At the same time, Linus Torvalds and company could
invent a fantastically wonderful tool which they created by mixing together
darcs and Linux source code, and they distribute this tool to the world. The
license that they would use would say "GPL v2".
These two things would simultaneously be possible if darcs were under
"GPL-plus-special-exception-for-CPL". Obviously it would not be possible for
all three source bases to be mixed together, because of the incompatibility
between Eclipse's CPL and Linux's GPLv2.
That's why the ascii art that I posted is correct with regard to the issue of
the proposed special exception to the license (but not with regard to the
issue of adopting code from one of the other two projects and making darcs
require that code for darcs to work).
If darcs is under GPL then the following three rules hold:
+--------------------+
| allowed |
+---------+ |+-------+ +-------+|
| Eclipse | || darcs | | Linux ||
| CPL | || GPL | | GPL ||
+---------+ |+-------+ +-------+|
+--------------------+
+----------------------+
| disallowed |
|+---------+ +-------+| +-------+
|| Eclipse | | darcs || | Linux |
|| CPL | | GPL || | GPL |
|+---------+ +-------+| +-------+
+----------------------+
+---------------------------------+
| disallowed |
|+---------+ +-------+ +-------+|
|| Eclipse | | darcs | | Linux ||
|| CPL | | GPL | | GPL ||
|+---------+ +-------+ +-------+|
+---------------------------------+
If darcs is under GPL-plus-special-exception-for-CPL then the following three
rules hold:
+--------------------+
| allowed |
+---------+ |+-------+ +-------+|
| Eclipse | || darcs | | Linux ||
| CPL | || GPL+se| | GPL ||
+---------+ |+-------+ +-------+|
+--------------------+
+----------------------+
| allowed |
|+---------+ +-------+| +-------+
|| Eclipse | | darcs || | Linux |
|| CPL | | GPL+se|| | GPL |
|+---------+ +-------+| +-------+
+----------------------+
+---------------------------------+
| disallowed |
|+---------+ +-------+ +-------+|
|| Eclipse | | darcs | | Linux ||
|| CPL | | GPL+se| | GPL ||
|+---------+ +-------+ +-------+|
+---------------------------------+
Regards,
Zooko
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