Consider GNU Objective C. NeXT initially wanted to make this front
end proprietary; they proposed to release it as .o files, and let
users link them with the rest of GCC, thinking this might be a way
around the GPL's requirements. But our lawyer said that this would
not evade the requirements, that it was not allowed. And so they
made the Objective C front end free software.
Specifically, as I've spoken about in my many talks on GPL
compliance, the earliest publicly discussed major GPL violation
was by NeXT computing when Steve Jobs attempted and failed (thanks
to RMS' GPL enforcement work) to make the Objective C front-end to
GCC proprietary. Everything for everyone involved would have gone
quite differently if that enforcement effort had failed.
They're basically achieving their dream now with Swift, though. I don't expect them to ever release that source. I don't think there is any precedent for Apple releasing source code after the fact.
