### Describe the feature you would like to be implemented: This proposal contains a meta-template implementation of DenseBase concatenation supporting arbitrary DenseBase expressions. There is a C++14 version to fit the next Eigen release and a cleaner C++17 version. ### Would such a feature be useful for other users? Why? It's a pretty classical thing to do :smile: ### Implementation of the requested feature: https://godbolt.org/z/fdafe9c5z