Analysis of information sources in references of the Wikipedia article "License proliferation" in English language version.
Apache 2 software can therefore be included in GPLv3 projects, because the GPLv3 license accepts our software into GPLv3 works. However, GPLv3 software cannot be included in Apache projects. The licenses are incompatible in one direction only, and it is a result of ASF's licensing philosophy and the GPLv3 authors' interpretation of copyright law.
At the time, the decision seemed sensible in the face of a deadlock. But now, GPLv2 is used for 42.5% of free software, and GPLv3 for less than 6.5%, according to Black Duck Software.
Copyleft is the main source of compatibility problems
The licences for distributing free or open source software (FOSS) are divided in two families: permissive and copyleft. Permissive licences (BSD, MIT, X11, Apache, Zope) are generally compatible and interoperable with most other licences, tolerating to merge, combine or improve the covered code and to re-distribute it under many licences (including non-free or "proprietary").
No. Some of the requirements in GPLv3, such as the requirement to provide Installation Information, do not exist in GPLv2. As a result, the licenses are not compatible: if you tried to combine code released under both these licenses, you would violate section 6 of GPLv2. However, if code is released under GPL "version 2 or later," that is compatible with GPLv3 because GPLv3 is one of the options it permits.
Viral property stimulates proliferation of licenses and contributes to the "GPL-enforced nightmare" -- a situation when many other licenses are logically incompatible with the GPL and make life unnecessary difficult for developers working in the Linux environment (KDE is a good example here, Python is a less known example). I think that this petty efforts to interpret GPL as a "holy text" are non-productive discussion that does not bring us anywhere. And they directly contributed to the proliferation of different "free software" licenses.
GPLv3 broke "the" GPL into incompatible forks that can't share code.
[...]since the FSF is proposing to shift all of its projects to GPLv3 and apply pressure to every other GPL licensed project to move, we foresee the release of GPLv3 portends the Balkanisation of the entire Open Source Universe upon which we rely.
Permissive licensing simplifies things One reason the business world, and more and more developers [...], favor permissive licenses is in the simplicity of reuse. The license usually only pertains to the source code that is licensed and makes no attempt to infer any conditions upon any other component, and because of this there is no need to define what constitutes a derived work. I have also never seen a license compatibility chart for permissive licenses; it seems that they are all compatible.
The License Compatibility Clusterfuck - When the GPL is involved the complexities of licensing becomes a non fun version of a riddle. So many things to consider and so many interactions to consider. And that GPL incompatibilities are still an issue that actively effects people is something many appear to forget. For instance one would think that the incompatibility of the GPLv2 with the Apache Software License 2.0 should be a thing of the past now that everything upgrades to GPLv3, but it turns out that enough people are either stuck with GPLv2 only or do not agree with the GPLv3 that some Apache Software licensed projects are required to migrate. For instance Twitter's Bootstrap is currently migrating from ASL2.0 to MIT precisely because some people still need GPLv2 compatibility. Among those projects that were affected were Drupal, WordPress, Joomla, the MoinMoin Wiki and others. And even that case shows that people don't care that much about licenses any more as Joomla 3 just bundled bootstrap even though they were not licenses in a compatible way (GPLv2 vs ASL 2.0). The other traditional case of things not being GPL compatible is the OpenSSL project which has a license that does not go well with the GPL. That license is also still incompatible with the GPLv3. The whole ordeal is particularly interesting as some not so nice parties have started doing license trolling through GPL licenses.
In the end, GPLv3 constitutes license proliferation.
Copyleft is the main source of compatibility problems
The licences for distributing free or open source software (FOSS) are divided in two families: permissive and copyleft. Permissive licences (BSD, MIT, X11, Apache, Zope) are generally compatible and interoperable with most other licences, tolerating to merge, combine or improve the covered code and to re-distribute it under many licences (including non-free or "proprietary").
Viral property stimulates proliferation of licenses and contributes to the "GPL-enforced nightmare" -- a situation when many other licenses are logically incompatible with the GPL and make life unnecessary difficult for developers working in the Linux environment (KDE is a good example here, Python is a less known example). I think that this petty efforts to interpret GPL as a "holy text" are non-productive discussion that does not bring us anywhere. And they directly contributed to the proliferation of different "free software" licenses.
