std::extents<IndexType,Extents...>:: rank_dynamic

#include <iostream>
#include <mdspan>
int main()
{
    std::extents<int, 1, 2> e1;
    std::extents<int, 3, 4, std::dynamic_extent> e2(5);
    std::extents<int, std::dynamic_extent, 7, std::dynamic_extent> e3(6, 8);
    std::cout << e1.rank_dynamic() << ", "
              << e2.rank_dynamic() << ", "
              << e3.rank_dynamic() << '\n';
}