Stated more precisely: a non-GPL-compatible license for an extension means
that the extension can never be distributed with core.
The idea that deployment of software on a server entails license
obligations is a GPLv3 feature; mediawiki is licensed under the GPL v2 ("or
later" for theoretical redistribution purposes).
Presumably deployment of a GPLv3-only or [[Affero GPL]] extension on a WMF
server might be more problematic, iff WMF were deploying non-GPL-compatible
extensions (I don't know whether that's the case one way or another). But
that's rather orthogonal to the "openness" of the source.
Anyway, we could devolve into a flame war and/or "discussion" of the merits
and disadvantages of various software licenses rather easily, so I'm
suggesting that we limit further discussion on this thread absent a more
focused question from Jeroen.
--scott
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(
http://cscott.net)