Stefan Monnier
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.
So it's okay to reimplement the GPL'd API with a different license -- this is apparently not a derived work -- but making use of it is not allowed?
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.
So the ability to *use* is what is crucial here? This is very surprising to me in the context of copyright law. At any rate you'd be able to use the source code of Y for teaching, bedtime reading, or whatever. Normally, I'd expect to write some text and publish it under any license I wish, the exeption being if I incorporate somebody elses work. I also thought merely conforming to an API didn't count as such incorporation, since this would mean glibc would be GPL (using the kernel API), which in turn would make GHC GPL (linking to glibc).
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
This sounds very interesting, but unfortunately, I am unable to google any details. Links? -k -- If I haven't seen further, it is by standing in the footprints of giants