2010/3/4 Stefan Monnier
The next question that comes to mind is thus: What if a new library X' released under BSD or MIT license implements the X API (making possible to compile Y against it)? Can such a new library X' be licensed under something else than the GPL (we guess Yes because we don't think it is possible to license the API itself)?
Yes.
Why should the existence of X' make any difference for the author of Y?
Because the existence of X' makes it possible to use Y without using X. The order in which X and X' come to exist doesn't matter.
This exact scenario took place for the GMP library, whose API was reimplemented as fgmp, specifically so that a user of the GMP library could release their code under a different library than the GPL.
The thing is that the new X' library can provide the same API while not being very useful (bug, performance, whatever). And in this case, it is trivial to make that new X'. So I don't understand why the answer was no in the first place. They also said no for the second question, which was asking about some possibility to make it legal to not license Y under GPL, and we are saying that providing a new implementation is such a possibility. So it is still either unclear, or either very constraining. Cheers, Thu