The primary reason for this updated minimum version is to minimize developer burden for the test cases.
Newer versions of cmake (greater than 3.0) add several convenient directives that require less cmake code.
Versions of cmake greater than 3.5 provide increased support for CXX11 standards compliance.
This issue is primarily to start a discussion about how much development burden should be placed on maintaining such long backward compatibility.
pkgs.org tells me that the available versions of CMake in the Linux distributions it knows about are as follows:
Arch Linux: 3.15.2
CentOS: 126.96.36.199 (CentOS 7 and 6)
Debian: 3.13.4 (Debian 10), 3.7.2 (Debian 9), 3.0.2 (Debian 8)
Fedora: 3.14.5 (Fedora 30 and 29)
Mageia: 3.14.3 (Mageia 7), 3.10.2 (Mageia 6)
OpenMandriva: 3.14.5 (OpenMandriva 4.0), 3.11.4 (OpenMandriva 3.0)
openSUSE Leap: 3.10.2 (this is the release-based distribution version of openSUSE)
openSUSE Tumbleweed: 3.13.4 (this is the rolling release of openSUSE)
Slackware: 3.5.2 (Slackware 14.2), 2.8.12 (Slackware 14.1), 2.8.8 (Slackware 14.0)
Ubuntu: 3.13.4 (Ubuntu 19.04), 3.10.2 (Ubuntu 18.04), 3.5.1 (Ubuntu 16.04), 2.8.12/3.5.1 (Ubuntu 14.04; 3.5.1 is available as the cmake3 package)
In addition to the above, RHEL 8 has 3.11.4; previous versions of RHEL carry the same version as CentOS (as you’d expect). EPEL carries a cmake3 package providing version 3.13.5 for CentOS and RHEL 7, and version 3.6.1 for CentOS and RHEL 6.
For non-Linux distributions:
macOS (via Homebrew): 3.15.2
The versions above are the latest version available in each release of the given distribution, not necessarily the default version — I expect most users to be comfortable enough keeping their distribution up to date within a given release.
I don’t know what’s needed to cover 90% of the installed base. If you stick to the latest version of all distributions above, then 3.5.2 covers everything apart from CentOS, and 3.10.2 covers everything apart from CentOS and Slackware. The CentOS situation should be temporary anyway, since CentOS 8 is forthcoming, and can be worked around by using EPEL. If you want to include releases which are still in wide use, then you’re down to 188.8.131.52 since CentOS and RHEL have a large installed base (but again, see EPEL), or 3.5.1 if you want to ignore that but still include popular releases of Debian and Ubuntu.
I suspect the most popular distribution on the desktop is Ubuntu, followed perhaps by Fedora (ignoring macOS and ChromeOS); on servers, RHEL, CentOS and other RHEL derivatives, Debian, Ubuntu, and proprietary distributions used by server hosts, in some order. But it’s impossible to get reliable data. (Distrowatch’s popularity figures reflect the popularity of Distrowatch’s pages on each distribution, not the usage of each distribution).
I agree that updating to at least cmake-3.0 should be no problem.
Also, no strong objections against 3.5 (We don't have an overview of what Eigen-users are using, the only realistic objection I see that might arise would be Debian 8 LTS users). We are already very generous in supporting old compilers, and cmake is actually not even needed for using Eigen (it mostly is required for building the tests and the documentation).
For Eigen-3.3, I'd keep it as is (since we hopefully release 3.4 in the near future anyway) -- but I don't have strong objections either for upgrading this as well. If we do, we should try to equalize the cmake files between 3.3 and 3.4/devel as much as possible.
I'll send a mail to the public mailing list soon, to hear if any reasonable objections exist.
just a short comment: PR592 by Christoph (https://bitbucket.org/eigen/eigen/pull-requests/592) technically also requires at least CMake 3.1.
I'd really welcome an upgrade.
It's worth noting that CMake is one of the easiest programs to install a recent version for, if the version in the OS's package manager is too low. You can just wget the appropriate binary from cmake.org, unzip, (optionally add to the PATH), and run /home/my/cmake/bin/cmake. No admin rights or dependencies required.
A +1 from me to bump the minimum required cmake version to the highest needed / highest that makes sense (personally on most of my projects I require >=3.8.2 or even >=3.12 as there are many C++17 and MSVC fixes as well as improvements to targets and properties).
I'm 100% fine with requiring 3.10 which is two years old, especially for the devel branch.
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