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I considered this issue in my current project (https://github.com/uonrobotics/ctmd) and redefined mdarray to use the std::array container when rank_dynamic() is 0.

namespace detail {

template <extents_c extent_t>
[[nodiscard]] inline constexpr size_t static_mdarray_size() noexcept {
    if constexpr (extent_t::rank() == 0) {
        return 0;

    } else {
        return []<size_t... Is>(std::index_sequence<Is...>) {
            return (extent_t::static_extent(Is) * ...);
        }(std::make_index_sequence<extent_t::rank()>{});
    }
}

} // namespace detail

template <size_t Rank, class IndexType = size_t>
using dims = dextents<IndexType, Rank>

template <typename T, extents_c extent_t>
using mdarray = std::conditional_t<
    extent_t::rank_dynamic() == 0,
    std::experimental::mdarray<
        T, extent_t, layout_right,
        std::array<T, detail::static_mdarray_size<extent_t>()>>,
    std::experimental::mdarray<T, extent_t, layout_right, std::vector<T>>>;

With this definition, the mdarray can be instantiated as a constexpr when the rank is static, for example:

constexpr auto a = mdarray<T, extents<size_t, 2, 1, 2>>{std::array<T, 4>{1, 2, 3, 4}};

Alternatively, it can be created as a non-constexpr object with dynamic extents:

const auto a = mdarray<T, dims<3>>{std::vector<T>{1, 2, 3, 4}, dims<3>{2, 1, 2}};

Is this approach helpful for your problem?

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