mp_units::quantity_spec<QS, Eq, Args...>::operator()

template<
    typename Self,
    typename FwdValue,
    RepresentationOf<Self{}> Value = std::remove_cvref_t<FwdValue>>
requires (detail::explicitly_convertible_to_dimensionless(Self{}))
[[nodiscard]]
constexpr
Quantity auto
operator()(this
    Self,
    FwdValue&& val);