The Eigen maintainers met today, and have decided to make C++14 the minimum supported language standard on the master branch. (
https://en.cppreference.com/w/cpp/14)
Because of the change i language standard, and because we likely want to break existing APIs as we clean up and merge new features in the coming period, the next version of Eigen will most likely be called version
4.0.0.
If your project is unable to compile with c++14, you will have to stay with Eigen 3.4.x. The 3.4.x branch will be frozen in terms of features, but will continue to be maintained and receive bug fixes. If your project is in the (hopefully unlikely) situation you cannot use c++14 but also want to add or use new features not in 3.4.x, let us know.
This change means that the we will change the minimum compiler versions supported by Eigen to the earliest one supporting all c++14 core language and library features. As far as we can tell, that means GCC 5, Clang 3.4, and MSVC 19.10. We will also drop support CUDA versions older than 9.