mp_units::operator+

template<
    std::derived_from<quantity> Q,
    RepresentationOf<quantity_spec> Value>
requires detail::DimensionlessOne<Q::reference> &&
             detail::InvokeResultOf<quantity_spec, std::plus<>, Rep, const Value&>
[[nodiscard]]
constexpr
Quantity auto
operator+(
    Value const& lhs,
    Q const& rhs);