Assuming you used the original source code to derive the detailed spec, then yes, that too is a derivative work.
The "viral" nature of that sort of license has bothered me for a long time. It's always been simultaneously overly far reaching and impossible to realistically enforce.
But here's an interesting question:
If you do not execute the code - did you accept the license? Does simply reading it sufficiently to be able to write a spec bind you to that license? That seems a bit too much.
@tbortels @lcamtuf @bgalehouse @kevinr Copyright bound licenses work by exempting you from the blanket and default prohibition on copying.
So if you copy a work that has copyright restrictions according to copyright law, using the license is your only way of not infringing the law. It doesn’t matter if you ”accept” it or not.
If you are not copying, the license is irrelevant.
@wouter @revk Plagiarism is about presenting the work as your own, not where it came from. It can be loosely connected to copyright because of either legal or license demands for attribution, but even taking something that is public domain and presenting it as your own is plagiarism.
Where copyright is about the relation to people a work comes from, I would say plagiarism is about the relation to people you present a work to.
I think "fanfiction" is closer.
Although if it's close enough to be identical in all major respects, a "remake" is also accurate.
Having said that: software is weird because it does something. If I write code that faithfully implements an API - I haven't stolen or plagiarized or anything - I followed the spec.
If that spec was supposed to be secret or proprietary - then open sourcing the code was a bad idea, and the soul of GPL is to force sharing.
Anyway - hot mess. As things tend to get when you try to force other human beings, legally or otherwise, to do what you want them to do.