The D library "Mir GLAS" also does the eigen style optimizations, IIRC. It's not unique to C++ but yes Eigen is by far and away the most mature option in that

The D library "Mir GLAS" also does the eigen style optimizations, IIRC. It's not unique to C++ but yes Eigen is by far and away the most mature option in that space. D templates are significantly better than C++ ones in basically every way, but the work is already done in C++ so I can't be too boastful