dextents not available in std::experimental namespace

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.

Start by inspecting extents.hpp and the namespace definitions involved in the provided reproducer. Compile the example with the project headers, then verify that dextents is available through std::experimental alongside the other mdspan components.