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.

https://www.gnu.org/philosophy/pragmatic.html

    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.

http://ebb.org/bkuhn/blog/2014/01/26/llvm.html