A concept matching all quantities of the provided quantity spec
<mp-units/framework/quantity_concepts.h>
template<
    typename Q,
    auto QS>
concept QuantityOf = Quantity<Q> && QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(QS))> &&
                     ReferenceOf<MP_UNITS_NONCONST_TYPE(Q::reference), QS>;
Satisfied by all quantities with the reference satisfying ReferenceOf<QS>.