Ubuntu 20.04 LTS will be released soon, and I believe it’ll have CMake 3.16.3, so that increases the LTS lower bound significantly.
I strongly disagree with the sentiment that the build system already works so there’s no urgent need to improve it. I believe we should treat the build system like code, and the same ideas around refactoring apply. Our build system is a huge thorny mess; there’s tons of supported configurations, lots of dependencies which rely on targets being processed in a certain order, and so on. Some of that can be cleaned up without requiring a CMake version bump, but generator expressions in particular are a very powerful mechanism which enable targets to depend on each other without requiring a particular order for processing them (which is very fragile and easy to break). In turn, build system cleanups reduce bugs (and we get plenty of those), make it much easier to add features with confidence, and make the build system much more understandable. I think a policy of “we only upgrade CMake versions when it’s absolutely necessary to do so” would be pretty harmful for the health of the build system.
Huge +1 from me! This is not about cosmetics, this is about improving the health of the project. There are known bugs in our build that we cannot solve without the use of generator expressions (for example https://reviews.llvm.org/D68833).
Personally I’d really like LLVM to move to 3.15 which greatly expanded the generator expression support and would give us the biggest benefit.