Does reimplementing GPLv2 code in another language create a derivative work that also falls under the GPL?

Question

I am now working with some ~2005 C CLI code, licensed as GPLv2-only. I hope to do the following things:

1. Port the code to a more recent environment (e.g. port to Ubuntu 22.04) (I only do this to try to get it running on a modern machine for doing the following steps)
2. Write a library in a different language (e.g. C#) based on findings made when doing (1) with the functionality of (1) (Note: not porting, but coding the same features with (possibly) entirely different code, without looking at the original code or (1))
3. Port (2) back to C
4. Build CLIs for both (2) and (3) in their respective languages (i.e. C & C#)

For (1) I'm certain I need to license it as GPLv2-only, but for the others I'm uncertain, and my goal is make my software available under the MIT/Expat License. Am I required to make them all GPLv2-only?

Answer 1

Yes. A derivative is a derivative. Porting existing code to another language also counts as making a modification.

NOT a derivative is a so-called blackbox implementation. That is the case when you look only at the behaviour of the library WITHOUT looking at the code and re-implement that; this might be argued still to some degree if you take the interface definition but not the implementation and implement the code without resorting to looking at the implementation at all (this was important in the quarrels between google and oracle).

Prominent example very similar to your question is where this became an issue was within the mercurial ecosystem. There the developers cracked down on re-implementations in other languages which their authors wanted to distribute under a permissive license without the original mercurial developers consent. Some original discussion from 15 years back is here.