[jcranmer]

> I think it's also worth noting that "derivative work" is an established legal term

The last case I could find that substantially attacks the question of what constitutes a "derivative work" in the case of software was in 1994 (!), and I can't find any case that tries to answer it in the case of software libraries (whose only purpose is to be used in larger works). There are tests for determining "derivative work", but a) minor circuit split there and b) they're difficult to apply. In the context of what we're talking about here, it is not an established legal term. In such cases, courts are going to lean more heavily on consensus interpretation.

There's a not-well-understood interplay here between the rights of authors to control derivative works; extending analogies copyrightable and non-copyrightable elements of literary works (well-established) to software (far less so); fair use; and general contract principles (specifically, selling a product confers necessary rights to use it). Our current legal understanding of the situation is almost entirely based on an informal consensus rather than any case law, and there is wide potential for a single oddball case completely wrecking everything... like Google v Oracle.

> Determining what constitutes "intimiate enough" sounds pretty difficult and open to interpretation

I read that generally as suggesting a mens rea approach: if you're building IPC in such a way as to specifically avoid meeting license obligations, you should be considered in violation. If instead you're using it in a more well-defined and sanctioned API boundary, you're probably okay.