How to use C++ ranges to implement numpy.ndindex?

views::cartesian_product is already variadic, you just need to expand a pack into it.

template <typename... Ts>
inline auto NDIndex(Ts ... ds) {
  using namespace ranges;
  return views::cartesian_product(views::indices(ds)...);
}

// Usage
int main() {
    for (const auto[i1, i2] : NDIndex(5, 4)) {
    }
    for (const auto[i1, i2, i3] : NDIndex(5, 4, 7)) {
    }
}

This can be done this way

#include <range/v3/view/indices.hpp>
#include <range/v3/view/cartesian_product.hpp>


template <unsigned... Ind>
constexpr inline auto NDIndex() {
  using namespace ranges;
  return views::cartesian_product(views::indices(Ind)...);
}


int main() {

    for (const auto[i1, i2] : NDIndex<5, 4>()) {
    }


    for (const auto[i1, i2, i3] : NDIndex<5, 4, 7>()) {
    }
}

Live example