mp_units::quantity::operator Q_

template<
    typename Q_,
    QuantityLike Q = std::remove_cvref_t<Q_>>
requires detail::QuantityConstructibleFrom<detail::quantity_like_type<Q>, quantity>
[[nodiscard]]
constexpr
explicit(quantity_like_traits<Q>::explicit_export || !std::convertible_to<quantity<R, Rep>, detail::quantity_like_type<Q>>)
operator Q_() const noexcept(noexcept(quantity_like_traits<Q>::from_numerical_value(this‐>numerical_value_is_an_implementation_detail_)) && std::is_nothrow_copy_constructible_v<rep>);