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

template<
    typename Self,
    typename FwdQ,
    Quantity Q = std::remove_cvref_t<FwdQ>>
requires (mp_units::explicitly_convertible(Q::quantity_spec, Self{})) &&
            requires { typename quantity<make_reference(Self{}, Q::unit), typename Q::rep>; }
[[nodiscard]]
constexpr
Quantity auto
operator()(this
    Self,
    FwdQ&& q);