Note that this is heavily dependent on C++20. I'm not even caught up on C++17, so I was thrown off by statements like this:

    quantity q1 = 42 * J;

where quantity is a class template:

    template<Reference auto R,
        RepresentationOf<get_quantity_spec(R).character> Rep = double>
    class quantity;

I didn't know you could use auto in a template parameter. I gather that Reference is a concept, something else I've yet to explore. Just when I thought I'd gotten the hang of "modern" C++.