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

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

https://debusine.debian.net/debian/developers-gladk-eigen3/artifact/3757105/

Here is the most probably relevant part of the failing build log:
    ===========================================

Dear maintainers,

eigen3 5.0 (libeigen3-dev 5.0) is now in unstable, so these FTBFS bugs are
release-critical.  I am therefore raising their severity to serious.

Regards,

Anton