The Free Software Foundation writes the following about the EUPL 1.2 (https://www.gnu.org/licenses/license-list.html#EUPL12):
This is a free software license. By itself, it has a copyleft comparable to the GPL’s, and incompatible with it. However, it gives recipients ways to relicense the work under the terms of other selected licenses, and some of those—the Eclipse Public License and the Common Public License in particular—only provide a weaker copyleft. Thus, developers can’t rely on this license to provide a strong copyleft.
The EUPL allows relicensing to GPLv2, because that is listed as one of the alternative licenses that users may convert to. It also, indirectly, allows relicensing to GPL version 3, because there is a way to relicense to the CeCILL v2, and the CeCILL v2 gives a way to relicense to any version of the GNU GPL.
To do this two-step relicensing, you need to first write a piece of code which you can license under the CeCILL v2, or find a suitable module already available that way, and add it to the program. Adding that code to the EUPL-covered program provides grounds to relicense it to the CeCILL v2. Then you need to write a piece of code which you can license under the GPLv3-or-later, or find a suitable module already available that way, and add it to the program. Adding that code to the CeCILL-covered program provides grounds to relicense it to GPLv3-or-later.
The fact that re-licensing from EUPL to GPL is so cumbersome (and therefor off-putting to independent developers), and that at the same time it allows for re-licensing to weaker copyleft (i.e. for derivative works to be more proprietary, so to speak), makes me not want to use it.
Highly interesting. However:
So these are the parts of the EUPL 1.2 that are most relevant to copyleft:
Having read this section multiple times, also in different languages, I preliminarily believe that the following still remains possible:
Let’s say that some person or entity “A” has released some code under the EUPL.
Some other person or entity “B” creates a derivative work and distributes it (including all of A’s code) under the LGPL. This is allowed per the first sentence of the EUPL’s Compatibility clause above: “this Distribution or Communication can be done under the terms of this Compatible Licence”. Here B is a licensee of the EUPL-licenced work, and what the final part of the Compatibility clause (just like the text that you quoted) says is that B, being a licensee of a EUPL-licensed work, continues to be bound by all of the EUPL’s copyleft obligations. Fair enough.
Now some third person or entity “C” comes along, and takes just this re-distributed work, which is being distributed by B under the terms of just the LGPL. Here C has no obligations under the EUPL, because C is only dealing with code that is distributed by B under just the LGPL. That is, C is solely a licensee under the terms of the LGPL.
And thus the exploit would be: Corporation C pays some straw man company B to re-distribute A’s interesting EUPL code under the LGPL, so that corporation C can pick it up while only needing to comply with the weaker copyleft of the LGPL.