@wds15, we got a reply from the legal team at NumFOCUS:
There is no problem with combining the software as they propose. As I understand it, we have:
StanHeaders (BSD-3-Clause)
including
StanMath
including
RStan (GPLv3+).On compilation, StanHeaders and RStan are combined.
It is acceptable to combine GPLv3 software with BSD-3- Clause-licensed software. It’s a bit inaccurate to say the GPL license requires “relicensing” of third-party software that is already under a different license. The accurate statement is that any software combined with GPL software cannot have compliance requirements that are inconsistent with those in the GPL license. So as long as the third party license’s terms are consistent in all ways with the GPL, then they can be combined.
The requirements for the BSD-3-Clause are:
The copyright notice and a copy of the license must be included in source code.
The copyright notice and a copy of the license must be included in materials provided with a binary distribution.
One cannot use the name of the author of the code to promote products.
There is a waiver of warranty and liability.The GPLv3 license has these same requirements in Section 4, stating that copyright notices must be retained and a copy of the license must be provided. The GPLv3 also has similar warranty disclaimer language. So there is nothing in the 3 Clause BSD license that imposes any different requirements than what the GPL also requires, so they are considered “compatible.”
The Free Software Foundation, the license steward of the GPL license, lists the Modified BSD License (another name for the 3 Clause BSD license) under its category of “GPL-Compatible Free Software Licenses.” So the Free Software Foundation believes it’s acceptable to combine them.
Conceptually, when you do combine them you can think of it as having passed along fewer rights in the BSD software than you received, which one is allowed to do with permissive licenses. Another way to think of it is that the BSD-licensed software now has a new layer of added restraints, for example a new duty to provide the source code. However in reality no one thinks of it these ways, because of course anyone can always just go to the original source for the BSD code and get a copy without any added restrictions. The Software Freedom Law Center also publishes a guide on maintaining permissively licensed software in a GPL distribution if you’re interested in maintaining a more freely licensed version of permissively licensed software alongside the GPL code.
I hope that helps, and let me know if there are any follow-up questions or you want to have a call about it.
Does this help to move forward or would you like for me to arrange the call that is offered?