-Wdeprecated-anon-enum-enum-conversion warnings (c++20)

Eigen uses anonymous enums in arithmetic without explicit conversion. This is deprecated in c++20/c++2a, see here. I have attached a file which reproduces these warnings. Compile with "clang++ -std=c++2a -Wdeprecated-anon-enum-enum-conversion -I/path/to/eigen test.cpp" using clang version 10 or greater.