compiling Eigen with GCC 9.X and -std=c++20

Not sure whether it really qualifies as a bug as the main problem is the support for C++20 features in the compiler: When I compile Eigen trunk with GCC 9.3 and -std=c++20, I get the following error(https://godbolt.org/z/zbvGbM3xr):

opt/compiler-explorer/libs/eigen/vtrunk/Eigen/src/Core/util/Meta.h:238:14: error: 'std::ssize' has not been declared
  238 |   using std::ssize;
      |              ^~~~~

The error disappears for newer GCC versions. However, I had a look at https://eigen.tuxfamily.org/dox/TopicPreprocessorDirectives.html and thought the error could be fixed by using #define EIGEN_MAX_CPP_VER 17, sadly it had no effect as you can see in the compiler explorer example.