The GPL prevents you from shipping compiled binaries or modified source. A LibreDWG plugin to LibreCAD is likely to be easily distributable purely as source. F

The GPL prevents you from shipping compiled binaries or modified source. A LibreDWG plugin to LibreCAD is likely to be easily distributable purely as source. FSF doesn't like to talk about it (it gets into quite dangerous territory for them when talking about interpreted languages), but the reality is that GPL relies on copyright. Specifically, it relies on their ability to make a copyright claim to things you are trying to share. If they have no actual copyright claim to something you're distributing, they are shit out of luck. FSF and licensing@ really likes to pretend that they can prevent this. But they never give specifics and always talk in generalities. They are bluffing and have no actual legs to stand on. If they have no copyright claim to something, they have no actual power over it. At all. It's out of their GPL's grasp. Of course, the FSF also wants it both ways when convenient. For example, they want to co-opt Matlab's MEX API for GNU/Octave, and simultaneously prevent you from doing the same to their code. This is allowed because of their magical "compatibility" wand. Apparently, they may copy interfaces as they wish, but for nebulous reasons you are expected to trust them that you may not do the same. See also the existence of ZFS on Linux distributed as patches/source-code shims (not a FSF issue as FSF does not own the Linux kernel, but ultimately it's the same licensing issue).