How to cast simd_mask of type T to another type?

[torshepherd]

version / revision	Operating System	Compiler & Version	Compiler Flags	CPU

  GCC (trunk/all)          |      x86-64      |    GCC (trunk/all)        |  -mavx2      |

Testcase

using doublev = std::experimental::native_simd<double>;
using size_tv = std::experimental::rebind_simd_t<std::size_t, doublev>;

struct Bound
{
    std::vector<doublev> vals;
    std::vector<size_tv> idxs;
};

void min(Bound& a, const Bound& b) {
    for (std::size_t i = 0; i < a.vals.size(); ++i)
    {
        const auto otherIsLess = b.vals[i] < a.vals[i];
        std::experimental::where(otherIsLess, a.vals[i]) = b.vals[i];
        std::experimental::where(otherIsLess, a.idxs[i]) = b.idxs[i];
    }
}

Actual Results

This code doesn't compile (https://godbolt.org/z/666G5YTqK)

Expected Results

I couldn't find anywhere on cppreference indicating the correct pattern for this use case - how can I apply a mask to different SIMD types of different element_type's?

[mattkretz]

Well, it's an embarrassing omission in the Parallelism TS 2. The libstdc++ implementation provides two ways out:

where(otherIsLess.__cvt(), a.idxs[i]) = b.idxs[i];

or, the verbose variant (which, except for the __proposed namespace, the C++ committee would have okayed for the TS):

where(std::experimental::__proposed::static_simd_cast<typename size_tv::mask_type>(otherIsLess), a.idxs[i]) = b.idxs[i];

I recommend:

namespace stdx
{
  using namespace std::experimental;
  using namespace std::experimental::__proposed;
}

or simply use https://github.com/mattkretz/vir-simd.

I guess cppreference should discuss this issue in some form...

[mattkretz]

Added a Notes section to https://en.cppreference.com/w/cpp/experimental/simd/simd_cast.

[torshepherd]

Thank you for the response! By the way, this library is amazing. Thank you for your hard work