Rework our mdspan implementation

libcudacxx/include/cuda/std/__algorithm/comp_ref_type.h

@@ -74,10 +74,10 @@ struct __debug_less
 #ifdef _CCCL_ENABLE_DEBUG_MODE
 template <class _Comp>
 using __comp_ref_type = __debug_less<_Comp>;
-#else
+#else // ^^^ _LIBCUDACXX_ENABLE_DEBUG_MODE ^^^ / vvv !_LIBCUDACXX_ENABLE_DEBUG_MODE vvv
 template <class _Comp>
 using __comp_ref_type = _Comp&;
-#endif
+#endif // !_LIBCUDACXX_ENABLE_DEBUG_MODE
 
 _LIBCUDACXX_END_NAMESPACE_STD
 

libcudacxx/include/cuda/std/__fwd/mdspan.h

@@ -0,0 +1,73 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCUDACXX___FWD_MDSPAN_H
+#define _LIBCUDACXX___FWD_MDSPAN_H
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/std/__type_traits/void_t.h>
+
+#if _CCCL_STD_VER >= 2014
+
+_LIBCUDACXX_BEGIN_NAMESPACE_STD
+
+// Layout policy with a mapping which corresponds to FORTRAN-style array layouts
+struct layout_left
+{
+  template <class _Extents>
+  class mapping;
+};
+
+// Layout policy with a mapping which corresponds to C-style array layouts
+struct layout_right
+{
+  template <class _Extents>
+  class mapping;
+};
+
+// Layout policy with a unique mapping where strides are arbitrary
+struct layout_stride
+{
+  template <class Extents>
+  class mapping;
+};
+
+// [mdspan.layout.policy.reqmts]
+namespace __mdspan_detail
+{
+template <class _Layout, class _Extents, class = void>
+_CCCL_INLINE_VAR constexpr bool __is_valid_layout_mapping = false;
+
+template <class _Layout, class _Extents>
+_CCCL_INLINE_VAR constexpr bool
+  __is_valid_layout_mapping<_Layout, _Extents, void_t<typename _Layout::template mapping<_Extents>>> = true;
+} // namespace __mdspan_detail
+
+_LIBCUDACXX_END_NAMESPACE_STD
+
+#endif // _CCCL_STD_VER >= 2014
+
+#endif // _LIBCUDACXX___FWD_MDSPAN_H