@tannhaeuser requesting an assessment of the EUPL (strong or weak copyleft).
Recital (10) of Directive 2009/24/EC on the legal protection of computer programs reminds that the function of a computer program is to communicate and work together with other components of a computer system (a logical and, where appropriate, physical interconnection, done by interfaces). This is known as “interoperability”. Recital (15) of the same directive states that the reproduction of the source code that is indispensable to achieve interoperability does not require the authorisation of the right holder, provide it is done by a person having the right to use that code and provide this use does not conflicts with a normal exploitation of that source code. Therefore, it seems that the directive implements an exception to the author’s exclusive rights. The objective of this exception is to make it possible to connect all components of a computer system, including those of different authors or manufacturers, so that they can work together. Notwithstanding the “strong” copyleft provision of the licence covering one or more of these components, it seems therefore that the legitimate recipient can always link two programs or link (logically or statically) any received code with its own code without being restricted by the copyleft provisions. The recitals of the directive are not transposed as such in the various national laws, but it makes little doubt that the Court of Justice of the EU will find there a motivation for rejecting any claim that “linking makes derivative” or that “linking extends the coverage of a copyleft licence”. This does not invalidate the concept of copyleft, that remains valid on the covered code, but it invalidates the so-called “viral” pretention of some copyleft licensors to extend the coverage of their licence to any linked program.
In other words, the famous distinction between “strong” and “weak” copyleft has little reality according to the EU legal framework.