A discussion on combining CDDL and GPL code

You are actually agreeing with me here. The assertion that anything supposedly written to work in multiple environments is, by its very nature, a completely separate work from any specific work that it might interact with (like the Linux kernel) has little or no bearing on the derived work discussion because that discussion around the GPL is all about the running system.

This has previously been discussed in the context of the Nvidia drivers which supposedly use the same code as the ones used on Windows, and so people like to claim that the Nvidia drivers could never be derived works of Linux. But the derived works discussion is not about those drivers in their "inert" state.

When considering the GPL, I like to consider the "view source" scenario: you're running Linux combined with the ZFS implementation delivered by Canonical; Linux is GPL-licensed and thus imposes certain conditions about it being combined with other things and the source code for the result being available (for further distribution under the GPL's terms); therefore, the complete source code for that running system must be available to the end-user. Can this really be satisfied if the source of the ZFS implementation cannot actually be obtained and redistributed under the GPL?

Naturally, people may argue (just as they do with the Nvidia drivers) that it's the user who combines the works and that no-one is actually distributing the combination. Others may argue that the different parts of the running system do not comprise a derived work of one of those parts. Such matters I will happily leave to the lawyers to argue about.