Dear Maintainer,

your package depends on libeigen3-dev. The upstream released a
new major version of this mathematical library, 5.0.0. Some
basic information about new features and changes in this version
can be found here:

https://libeigen.gitlab.io/releases/5.0/

This is a major version change and may require code changes in packages
depending on eigen3. The key breaking changes include:

  - Minimum required C++ standard is now C++14
  - Changes in the build system integration
  - Some API name changes

lbfgspp FTBFS with libeigen3-dev 5.0.1. The full build log is available at:

https://debusine.debian.net/debian/developers-gladk-eigen3/artifact/3757053/

Here is the most probably relevant part of the failing build log:
    ===========================================
-- Performing Test WALL_SUPPORTED
-- Performing Test WALL_SUPPORTED - Success
CMake Error at CMakeLists.txt:41 (find_package):
  Could not find a configuration file for package "Eigen3" that is compatible
  with requested version "3.0".

  The following configuration files were considered but not accepted:

    /usr/share/eigen3/cmake/Eigen3Config.cmake, version: 5.0.1
      The version found is not compatible with the version requested.