Isolate cuda::std::extent forward declarations (#7936)

Author: davebayer

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

@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2026 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 _CUDA_STD___FWD_EXTENTS_H
+#define _CUDA_STD___FWD_EXTENTS_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/__cstddef/types.h>
+#include <cuda/std/__fwd/span.h>
+
+#include <cuda/std/__cccl/prologue.h>
+
+_CCCL_BEGIN_NAMESPACE_CUDA_STD
+
+template <class _IndexType, size_t... _Extents>
+class extents;
+
+template <class _Tp>
+inline constexpr bool __is_cuda_std_extents_v = false;
+template <class _IndexType, size_t... _Extents>
+inline constexpr bool __is_cuda_std_extents_v<extents<_IndexType, _Extents...>> = true;
+
+// Recursive helper classes to implement dextents alias for extents
+template <class _IndexType, size_t _Rank, class _Extents = extents<_IndexType>>
+struct __make_dextents;
+
+template <class _IndexType, size_t _Rank, class _Extents = extents<_IndexType>>
+using __make_dextents_t = typename __make_dextents<_IndexType, _Rank, _Extents>::type;
+
+template <class _IndexType, size_t _Rank, size_t... _ExtentsPack>
+struct __make_dextents<_IndexType, _Rank, extents<_IndexType, _ExtentsPack...>>
+{
+  using type = __make_dextents_t<_IndexType, _Rank - 1, extents<_IndexType, dynamic_extent, _ExtentsPack...>>;
+};
+
+template <class _IndexType, size_t... _ExtentsPack>
+struct __make_dextents<_IndexType, 0, extents<_IndexType, _ExtentsPack...>>
+{
+  using type = extents<_IndexType, _ExtentsPack...>;
+};
+
+// [mdspan.extents.dextents], alias template
+template <class _IndexType, size_t _Rank>
+using dextents = __make_dextents_t<_IndexType, _Rank>;
+
+template <size_t _Rank, class _IndexType = size_t>
+using dims = dextents<_IndexType, _Rank>;
+
+_CCCL_END_NAMESPACE_CUDA_STD
+
+#include <cuda/std/__cccl/epilogue.h>
+
+#endif // _CUDA_STD___FWD_MDSPAN_H

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

@@ -28,7 +28,7 @@
 #  pragma system_header
 #endif // no system header
 
-#include <cuda/std/__cstddef/types.h>
+#include <cuda/std/__fwd/extents.h>
 #include <cuda/std/__type_traits/void_t.h>
 
 #include <cuda/std/__cccl/prologue.h>
@@ -107,14 +107,6 @@ inline constexpr bool __is_cuda_std_layout_left_or_right_mapping_v =
 // template<>
 // inline constexpr bool __is_cuda_std_layout_left_mapping_v<layout_left_padded> = true;
 
-template <class _IndexType, size_t... _Extents>
-class extents;
-
-template <class _Tp>
-inline constexpr bool __is_cuda_std_extents_v = false;
-template <class _IndexType, size_t... _Extents>
-inline constexpr bool __is_cuda_std_extents_v<extents<_IndexType, _Extents...>> = true;
-
 _CCCL_END_NAMESPACE_CUDA_STD
 
 #include <cuda/std/__cccl/epilogue.h>

libcudacxx/include/cuda/std/__mdspan/extents.h

@@ -609,35 +609,6 @@ class extents : private __mdspan_detail::__maybe_static_array<_IndexType, size_t
 #endif // _CCCL_STD_VER <= 2017
 };
 
-// Recursive helper classes to implement dextents alias for extents
-namespace __mdspan_detail
-{
-template <class _IndexType, size_t _Rank, class _Extents = extents<_IndexType>>
-struct __make_dextents;
-
-template <class _IndexType, size_t _Rank, class _Extents = extents<_IndexType>>
-using __make_dextents_t = typename __make_dextents<_IndexType, _Rank, _Extents>::type;
-
-template <class _IndexType, size_t _Rank, size_t... _ExtentsPack>
-struct __make_dextents<_IndexType, _Rank, extents<_IndexType, _ExtentsPack...>>
-{
-  using type = __make_dextents_t<_IndexType, _Rank - 1, extents<_IndexType, dynamic_extent, _ExtentsPack...>>;
-};
-
-template <class _IndexType, size_t... _ExtentsPack>
-struct __make_dextents<_IndexType, 0, extents<_IndexType, _ExtentsPack...>>
-{
-  using type = extents<_IndexType, _ExtentsPack...>;
-};
-} // end namespace __mdspan_detail
-
-// [mdspan.extents.dextents], alias template
-template <class _IndexType, size_t _Rank>
-using dextents = __mdspan_detail::__make_dextents_t<_IndexType, _Rank>;
-
-template <size_t _Rank, class _IndexType = size_t>
-using dims = dextents<_IndexType, _Rank>;
-
 // nvcc cannot handle type conversions without this workaround
 struct __to_dynamic_extent
 {