[eigen] Should we require c++11 for Eigen tensor

[Rasmus Munk Larsen <rmlarsen@xxxxxxxxxx>]

Dear Eigen tensor users,

Today, various c++11 features are in use in many parts of Eigen tensor, while other parts compile with c++03 without warnings. This division is rather arbitrary and primarily dictated by what parts of the code gets compiled for the subset of tests that are enabled in c++03 mode:

https://bitbucket.org/eigen/eigen/src/default/unsupported/test/CMakeLists.txt#lines-112

A larger set of tests covering the majority of the code base are only compiled in c++11 mode:
https://bitbucket.org/eigen/eigen/src/c4368f48fef3737ef5a48604cfc63ce946b68616/unsupported/test/CMakeLists.txt#lines-179

I don't think a user can realistically use Eigen tensor and avoid the c++11 parts of the code. Therefore I would propose that we stop pretending that Eigen tensor supports c++03 and simply guard all the code with EIGEN_HAS_CXX11 or similar. This will then allow us to start cleaning up the code.

Please let me know if you have any objections to this.

Best,
  Rasmus