Add mdspan-style template parameter support for Kokkos::View

[Copilot]

Enable Kokkos::View to accept mdspan-style template parameters (extents, layout_left/right, SpaceAwareAccessor) as an alternative to classical syntax (double**, LayoutLeft, CudaSpace), maintaining full backward compatibility.

Changes

• Trait detection (Kokkos_Concepts.hpp): Added is_mdspan_extents_v, is_mdspan_layout_v, and is_mdspan_accessor_v to identify mdspan types

• Type converters (Kokkos_ViewTraits.hpp):

  • ArrayLayoutFromMDSpanLayout: Maps layout_left/right/stride to LayoutLeft/Right/Stride
  • MemorySpaceFromAccessor: Extracts memory space from SpaceAwareAccessor
  • SafeDataTypeFromExtents: Converts element type + extents to data type specification

• ViewTraits specializations: Added SFINAE specializations to process mdspan parameters alongside existing ArrayLayout/Space/MemoryTraits specializations

• Primary template enhancement: Modified to detect mdspan parameters and convert them to canonical Kokkos types

Example

// Classical syntax (unchanged)
Kokkos::View<double**, Kokkos::LayoutLeft, Kokkos::CudaSpace> v1;

// New mdspan-style syntax (intended)
Kokkos::View<double, 
             Kokkos::dextents<size_t, 2>, 
             Kokkos::layout_left,
             Kokkos::Impl::SpaceAwareAccessor<Kokkos::CudaSpace, 
                                               Kokkos::default_accessor<double>>> v2;

Known Issue

SFINAE specialization conflict occurs when processing multiple mdspan types in sequence (e.g., ViewTraits<void, layout_left, SpaceAwareAccessor>). The recursive template instantiation doesn't match existing specializations correctly. Requires refactoring the specialization pattern to handle combined mdspan parameter cases.

Original prompt

I want to work on overhauling Kokkos::View to enable users to specify template parameters in the style of of mdspan or BasicView, while maintaining backward compatibility. E.g. I want both Kokkos::View<double, Kokkos::dextents<int, 2>, Kokkos::layout_left, Kokkos::SpaceAwareAccessor<Kokkos::CudaSpace, Kokkos::default_accessor<double>>> and the classical Kokkos::View<double**, Kokkos::LayoutLeft, Kokkos::CudaSpace, Kokkos::MemoryTraits<Kokkos::Unmanaged>> be valid ways to specify a Kokkos Views.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.