Compatibility of `boost.multiprecision` Eigen support with `eigen-git-mirror/master`

[miquelramirez]

Hello @jzmaddock,

This is not so much a bug report as a heads up on what potentially could be a future incompatibility with newer versions of Eigen.

I have run into the following issue when using the latest revision of boost/multiprecision/eigen.hpp and Eigen master branch, under g++11 (with the C++17 mode enabled):

boost/multiprecision/eigen.hpp:247:12: error: redeclared with 2 template parameters
  247 |     struct conj_impl;
      |            ^~~~~~~~~
....
/usr/local/include/eigen3/Eigen/src/Core/MathFunctions.h:261:34: note: previous declaration ‘template<class Scalar> struct Eigen::internal::conj_impl’ used 1 template parameter
  261 | template<typename Scalar> struct conj_impl : conj_default_impl<Scalar> {};
      |                                  ^~~~~~~~~
...
boost/multiprecision/eigen.hpp:256:94: error: wrong number of template arguments (2, should be 1)
  256 | struct conj_impl<boost::multiprecision::detail::expression<tag, Arg1, Arg2, Arg3, Arg4>, true>
      |                                                                                              ^
In file included from /usr/local/include/eigen3/Eigen/Core:159,
                 from /usr/local/include/eigen3/Eigen/Dense:1,
....
/usr/local/include/eigen3/Eigen/src/Core/MathFunctions.h:261:34: note: provided for ‘template<class Scalar> struct Eigen::internal::conj_impl’
  261 | template<typename Scalar> struct conj_impl : conj_default_impl<Scalar> {};
      |                                  ^~~~~~~~~
/

Eigen::internal::conj_impl is currently defined by Eigen as a single argument template. Removing the extra template argument here and here produces a correct build, but I haven't run the boost tests.

Thanks very much for maintaining this very useful component of boost!

[jzmaddock]

That's strange, I don't see any changes here: https://gitlab.com/libeigen/eigen/-/blob/master/Eigen/src/Core/MathFunctions.h#L257 ?

[mglisse]

Link to the Eigen side of things: https://gitlab.com/libeigen/eigen/-/issues/2112 (and https://gitlab.com/libeigen/eigen/-/commit/c0eb5f89a406243f71eae0b705eba4437d9f8565).

[jzmaddock]

OK, good. Is there something that isn't an internal detail we should have specialized instead?