mp_units::quantity::quantity

template<
    auto R2,
    typename Rep2>
requires detail::QuantityConstructibleFrom<quantity, quantity<R2, Rep2>> && (equivalent(unit, get_unit(R2)))
constexpr
explicit(!mp_units::implicitly_convertible(get_quantity_spec(R2), quantity_spec) || !mp_units::implicitly_scalable<get_unit(R2), Rep2, unit, rep>)
quantity(quantity<R2, Rep2> const& q);