CW: heavy open source license geekery ahead.

One unfortunate difficulty with open source licensing is that some lawyers, including the FSF, consider the Apache License 2.0 incompatible with the GPL 2.0, which is to say that you can’t combined Apache 2.0-licensed code with GPL 2.0-licensed code and distribute the result. This is annoying because when choosing a permissive licence, we want people to use the more modern Apache 2.0 over the older BSD or MIT licenses, because it provides some measure of patent protection. And this incompatibility discourages people from doing that.

This was a concern for Mozilla when determining the correct licensing for Rust, and this is why the standard Rust license is a dual license – the choice of Apache 2.0 or MIT. The idea was that Apache 2.0 would be the normal license, but people could choose MIT if they wanted to combine “Rust license” code with GPL 2.0 code.

However, the LLVM project has now had notable open source attorney Heather Meeker come up with an exception to be added to the Apache 2.0 license to enable GPL 2.0 compatibility. This exception meets a number of important criteria for a legal fix for this problem:

It’s an additional permission, so is unlikely to affect the open source-ness of the license;

It doesn’t require the organization using it to take a position on the question of whether the two licenses are actually compatible or not;

It’s specific to the GPL 2.0, thereby constraining its effects to solving the problem.

Here it is:

—- Exceptions to the Apache 2.0 License: —- In addition, if you combine or link compiled forms of this Software with software that is licensed under the GPLv2 (“Combined Software”) and if a court of competent jurisdiction determines that the patent provision (Section 3), the indemnity provision (Section 9) or other Section of the License conflicts with the conditions of the GPLv2, you may retroactively and prospectively choose to deem waived or otherwise exclude such Section(s) of the License, but only in their entirety and only with respect to the Combined Software. —- end —-

It seems very well written to me; I wish it had been around when we were licensing Rust.