dextents not available in std::experimental namespace

[mhoemmen]

dextents is not available in the std::experimental namespace (by default), even though mdspan and other mdspan components are. Instead, dextents can only be found in the std namespace by default.

Here is an example.

// does NOT compile
//template <typename T, size_t dimensions = 1>
//using slice0 = stdex::mdspan<T, stdex::dextents<size_t, dimensions>, std::experimental::layout_stride>;

// compiles
template <typename T, size_t dimensions = 1>
using slice1 = stdex::mdspan<T, std::dextents<size_t, dimensions>, std::experimental::layout_stride>;

// compiles
template <typename T, size_t dimensions = 1>
using slice2 = std::mdspan<T, std::dextents<size_t, dimensions>, std::layout_stride>;

One of my colleagues verified today that the problem exists in the headers, not just in the generated godbolt single header. The following code in the example

namespace MDSPAN_IMPL_STANDARD_NAMESPACE {
  inline constexpr int foobar = 42;
}

suggests what I think is the problem, namely that extents.hpp doesn't appear to import extents into the std namespace.

[nmm0]

Hmm... stdex::mdspan shouldn't compile, we might have a stray using declaration somewhere. All of the stuff merged into the standard should be under std:: or, better yet, Kokkos:: (include <mdspan/mdspan.hpp> for that namespace).