include/mdspan - llvm-project/libcxx - Git at Google

 // -*- 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
 //
 //===---------------------------------------------------------------------===//

 /*

 // Overall mdspan synopsis

 namespace std {
   // [mdspan.extents], class template extents
   template<class IndexType, size_t... Extents>
     class extents;

   // [mdspan.extents.dextents], alias template dextents
   template<class IndexType, size_t Rank>
     using dextents = see below;

   // [mdspan.layout], layout mapping
   struct layout_left;
   struct layout_right;
   struct layout_stride;

   // [mdspan.accessor.default], class template default_accessor
   template<class ElementType>
     class default_accessor;

   // [mdspan.mdspan], class template mdspan
   template<class ElementType, class Extents, class LayoutPolicy = layout_right,
            class AccessorPolicy = default_accessor<ElementType>>
     class mdspan; // not implemented yet
 }

 // extents synopsis

 namespace std {
   template<class _IndexType, size_t... _Extents>
   class extents {
   public:
     using index_type = _IndexType;
     using size_type = make_unsigned_t<index_type>;
     using rank_type = size_t;

     // [mdspan.extents.obs], observers of the multidimensional index space
     static constexpr rank_type rank() noexcept { return sizeof...(_Extents); }
     static constexpr rank_type rank_dynamic() noexcept { return dynamic-index(rank()); }
     static constexpr size_t static_extent(rank_type) noexcept;
     constexpr index_type extent(rank_type) const noexcept;

     // [mdspan.extents.cons], constructors
     constexpr extents() noexcept = default;

     template<class _OtherIndexType, size_t... _OtherExtents>
       constexpr explicit(see below)
         extents(const extents<_OtherIndexType, _OtherExtents...>&) noexcept;
     template<class... _OtherIndexTypes>
       constexpr explicit extents(_OtherIndexTypes...) noexcept;
     template<class _OtherIndexType, size_t N>
       constexpr explicit(N != rank_dynamic())
         extents(span<_OtherIndexType, N>) noexcept;
     template<class _OtherIndexType, size_t N>
       constexpr explicit(N != rank_dynamic())
         extents(const array<_OtherIndexType, N>&) noexcept;

     // [mdspan.extents.cmp], comparison operators
     template<class _OtherIndexType, size_t... _OtherExtents>
       friend constexpr bool operator==(const extents&,
                                        const extents<_OtherIndexType, _OtherExtents...>&) noexcept;

   private:
     // libcxx note: we do not use an array here, but we need to preserve the as-if behavior
     // for example the default constructor must zero initialize dynamic extents
     array<index_type, rank_dynamic()> dynamic-extents{};                // exposition only
   };

   template<class... Integrals>
     explicit extents(Integrals...)
       -> see below;
 }

 // layout_left synopsis

 namespace std {
   template<class Extents>
   class layout_left::mapping {
   public:
     using extents_type = Extents;
     using index_type = typename extents_type::index_type;
     using size_type = typename extents_type::size_type;
     using rank_type = typename extents_type::rank_type;
     using layout_type = layout_left;

     // [mdspan.layout.right.cons], constructors
     constexpr mapping() noexcept = default;
     constexpr mapping(const mapping&) noexcept = default;
     constexpr mapping(const extents_type&) noexcept;
     template<class OtherExtents>
       constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
         mapping(const mapping<OtherExtents>&) noexcept;
     template<class OtherExtents>
       constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
         mapping(const layout_right::mapping<OtherExtents>&) noexcept;
     template<class OtherExtents>
       constexpr explicit(extents_type::rank() > 0)
         mapping(const layout_stride::mapping<OtherExtents>&) noexcept;

     constexpr mapping& operator=(const mapping&) noexcept = default;

     // [mdspan.layout.right.obs], observers
     constexpr const extents_type& extents() const noexcept { return extents_; }

     constexpr index_type required_span_size() const noexcept;

     template<class... Indices>
       constexpr index_type operator()(Indices...) const noexcept;

     static constexpr bool is_always_unique() noexcept { return true; }
     static constexpr bool is_always_exhaustive() noexcept { return true; }
     static constexpr bool is_always_strided() noexcept { return true; }

     static constexpr bool is_unique() noexcept { return true; }
     static constexpr bool is_exhaustive() noexcept { return true; }
     static constexpr bool is_strided() noexcept { return true; }

     constexpr index_type stride(rank_type) const noexcept;

     template<class OtherExtents>
       friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;

   private:
     extents_type extents_{};    // exposition only
   };
 }

 // layout_right synopsis

 namespace std {
   template<class Extents>
   class layout_right::mapping {
   public:
     using extents_type = Extents;
     using index_type = typename extents_type::index_type;
     using size_type = typename extents_type::size_type;
     using rank_type = typename extents_type::rank_type;
     using layout_type = layout_right;

     // [mdspan.layout.right.cons], constructors
     constexpr mapping() noexcept = default;
     constexpr mapping(const mapping&) noexcept = default;
     constexpr mapping(const extents_type&) noexcept;
     template<class OtherExtents>
       constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
         mapping(const mapping<OtherExtents>&) noexcept;
     template<class OtherExtents>
       constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
         mapping(const layout_left::mapping<OtherExtents>&) noexcept;
     template<class OtherExtents>
       constexpr explicit(extents_type::rank() > 0)
         mapping(const layout_stride::mapping<OtherExtents>&) noexcept;

     constexpr mapping& operator=(const mapping&) noexcept = default;

     // [mdspan.layout.right.obs], observers
     constexpr const extents_type& extents() const noexcept { return extents_; }

     constexpr index_type required_span_size() const noexcept;

     template<class... Indices>
       constexpr index_type operator()(Indices...) const noexcept;

     static constexpr bool is_always_unique() noexcept { return true; }
     static constexpr bool is_always_exhaustive() noexcept { return true; }
     static constexpr bool is_always_strided() noexcept { return true; }

     static constexpr bool is_unique() noexcept { return true; }
     static constexpr bool is_exhaustive() noexcept { return true; }
     static constexpr bool is_strided() noexcept { return true; }

     constexpr index_type stride(rank_type) const noexcept;

     template<class OtherExtents>
       friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;

   private:
     extents_type extents_{};    // exposition only
   };
 }

 // layout_stride synopsis

 namespace std {
   template<class Extents>
   class layout_stride::mapping {
   public:
     using extents_type = Extents;
     using index_type = typename extents_type::index_type;
     using size_type = typename extents_type::size_type;
     using rank_type = typename extents_type::rank_type;
     using layout_type = layout_stride;

   private:
     static constexpr rank_type rank_ = extents_type::rank();    // exposition only

   public:
     // [mdspan.layout.stride.cons], constructors
     constexpr mapping() noexcept;
     constexpr mapping(const mapping&) noexcept = default;
     template<class OtherIndexType>
       constexpr mapping(const extents_type&, span<OtherIndexType, rank_>) noexcept;
     template<class OtherIndexType>
       constexpr mapping(const extents_type&, const array<OtherIndexType, rank_>&) noexcept;

     template<class StridedLayoutMapping>
       constexpr explicit(see below) mapping(const StridedLayoutMapping&) noexcept;

     constexpr mapping& operator=(const mapping&) noexcept = default;

     // [mdspan.layout.stride.obs], observers
     constexpr const extents_type& extents() const noexcept { return extents_; }
     constexpr array<index_type, rank_> strides() const noexcept { return strides_; }

     constexpr index_type required_span_size() const noexcept;

     template<class... Indices>
       constexpr index_type operator()(Indices...) const noexcept;

     static constexpr bool is_always_unique() noexcept { return true; }
     static constexpr bool is_always_exhaustive() noexcept { return false; }
     static constexpr bool is_always_strided() noexcept { return true; }

     static constexpr bool is_unique() noexcept { return true; }
     constexpr bool is_exhaustive() const noexcept;
     static constexpr bool is_strided() noexcept { return true; }

     constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; }

     template<class OtherMapping>
       friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept;

   private:
     extents_type extents_{};                    // exposition only
     array<index_type, rank_> strides_{};        // exposition only
   };
 }

 // default_accessor synopsis

 namespace std {
   template<class ElementType>
   struct default_accessor {
     using offset_policy = default_accessor;
     using element_type = ElementType;
     using reference = ElementType&;
     using data_handle_type = ElementType*;

     constexpr default_accessor() noexcept = default;
     template<class OtherElementType>
       constexpr default_accessor(default_accessor<OtherElementType>) noexcept;
     constexpr reference access(data_handle_type p, size_t i) const noexcept;
     constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
   };
 }

 // mdspan synopsis

 namespace std {
   template<class ElementType, class Extents, class LayoutPolicy = layout_right,
            class AccessorPolicy = default_accessor<ElementType>>
   class mdspan {
   public:
     using extents_type = Extents;
     using layout_type = LayoutPolicy;
     using accessor_type = AccessorPolicy;
     using mapping_type = typename layout_type::template mapping<extents_type>;
     using element_type = ElementType;
     using value_type = remove_cv_t<element_type>;
     using index_type = typename extents_type::index_type;
     using size_type = typename extents_type::size_type;
     using rank_type = typename extents_type::rank_type;
     using data_handle_type = typename accessor_type::data_handle_type;
     using reference = typename accessor_type::reference;

     static constexpr rank_type rank() noexcept { return extents_type::rank(); }
     static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
     static constexpr size_t static_extent(rank_type r) noexcept
       { return extents_type::static_extent(r); }
     constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); }

     // [mdspan.mdspan.cons], constructors
     constexpr mdspan();
     constexpr mdspan(const mdspan& rhs) = default;
     constexpr mdspan(mdspan&& rhs) = default;

     template<class... OtherIndexTypes>
       constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts);
     template<class OtherIndexType, size_t N>
       constexpr explicit(N != rank_dynamic())
         mdspan(data_handle_type p, span<OtherIndexType, N> exts);
     template<class OtherIndexType, size_t N>
       constexpr explicit(N != rank_dynamic())
         mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);
     constexpr mdspan(data_handle_type p, const extents_type& ext);
     constexpr mdspan(data_handle_type p, const mapping_type& m);
     constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);

     template<class OtherElementType, class OtherExtents,
              class OtherLayoutPolicy, class OtherAccessorPolicy>
       constexpr explicit(see below)
         mdspan(const mdspan<OtherElementType, OtherExtents,
                             OtherLayoutPolicy, OtherAccessorPolicy>& other);

     constexpr mdspan& operator=(const mdspan& rhs) = default;
     constexpr mdspan& operator=(mdspan&& rhs) = default;

     // [mdspan.mdspan.members], members
     template<class... OtherIndexTypes>
       constexpr reference operator[](OtherIndexTypes... indices) const;
     template<class OtherIndexType>
       constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
     template<class OtherIndexType>
       constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

     constexpr size_type size() const noexcept;
     [[nodiscard]] constexpr bool empty() const noexcept;

     friend constexpr void swap(mdspan& x, mdspan& y) noexcept;

     constexpr const extents_type& extents() const noexcept { return map_.extents(); }
     constexpr const data_handle_type& data_handle() const noexcept { return ptr_; }
     constexpr const mapping_type& mapping() const noexcept { return map_; }
     constexpr const accessor_type& accessor() const noexcept { return acc_; }

     // per LWG-4021 "mdspan::is_always_meow() should be noexcept"
     static constexpr bool is_always_unique() noexcept
       { return mapping_type::is_always_unique(); }
     static constexpr bool is_always_exhaustive() noexcept
       { return mapping_type::is_always_exhaustive(); }
     static constexpr bool is_always_strided() noexcept
       { return mapping_type::is_always_strided(); }

     constexpr bool is_unique() const
       { return map_.is_unique(); }
     constexpr bool is_exhaustive() const
       { return map_.is_exhaustive(); }
     constexpr bool is_strided() const
       { return map_.is_strided(); }
     constexpr index_type stride(rank_type r) const
       { return map_.stride(r); }

   private:
     accessor_type acc_;         // exposition only
     mapping_type map_;          // exposition only
     data_handle_type ptr_;      // exposition only
   };

   template<class CArray>
     requires(is_array_v<CArray> && rank_v<CArray> == 1)
     mdspan(CArray&)
       -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;

   template<class Pointer>
     requires(is_pointer_v<remove_reference_t<Pointer>>)
     mdspan(Pointer&&)
       -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;

   template<class ElementType, class... Integrals>
     requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
     explicit mdspan(ElementType*, Integrals...)
       -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;            // until C++26
   template<class ElementType, class... Integrals>
     requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
     explicit mdspan(ElementType*, Integrals...)
       -> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>;  // since C++26

   template<class ElementType, class OtherIndexType, size_t N>
     mdspan(ElementType*, span<OtherIndexType, N>)
       -> mdspan<ElementType, dextents<size_t, N>>;

   template<class ElementType, class OtherIndexType, size_t N>
     mdspan(ElementType*, const array<OtherIndexType, N>&)
       -> mdspan<ElementType, dextents<size_t, N>>;

   template<class ElementType, class IndexType, size_t... ExtentsPack>
     mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&)
       -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;

   template<class ElementType, class MappingType>
     mdspan(ElementType*, const MappingType&)
       -> mdspan<ElementType, typename MappingType::extents_type,
                 typename MappingType::layout_type>;

   template<class MappingType, class AccessorType>
     mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
            const AccessorType&)
       -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
                 typename MappingType::layout_type, AccessorType>;
 }
 */

 #ifndef _LIBCPP_MDSPAN
 #define _LIBCPP_MDSPAN

 #include <__config>
 #include <__fwd/mdspan.h>
 #include <__mdspan/default_accessor.h>
 #include <__mdspan/extents.h>
 #include <__mdspan/layout_left.h>
 #include <__mdspan/layout_right.h>
 #include <__mdspan/layout_stride.h>
 #include <__mdspan/mdspan.h>
 #include <version>

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

 #endif // _LIBCPP_MDSPAN
	// -- 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
	//
	//===---------------------------------------------------------------------===//

	/*

	// Overall mdspan synopsis

	namespace std {
	// [mdspan.extents], class template extents
	template<class IndexType, size_t... Extents>
	class extents;

	// [mdspan.extents.dextents], alias template dextents
	template<class IndexType, size_t Rank>
	using dextents = see below;

	// [mdspan.layout], layout mapping
	struct layout_left;
	struct layout_right;
	struct layout_stride;

	// [mdspan.accessor.default], class template default_accessor
	template<class ElementType>
	class default_accessor;

	// [mdspan.mdspan], class template mdspan
	template<class ElementType, class Extents, class LayoutPolicy = layout_right,
	class AccessorPolicy = default_accessor<ElementType>>
	class mdspan; // not implemented yet
	}

	// extents synopsis

	namespace std {
	template<class _IndexType, size_t... _Extents>
	class extents {
	public:
	using index_type = _IndexType;
	using size_type = make_unsigned_t<index_type>;
	using rank_type = size_t;

	// [mdspan.extents.obs], observers of the multidimensional index space
	static constexpr rank_type rank() noexcept { return sizeof...(_Extents); }
	static constexpr rank_type rank_dynamic() noexcept { return dynamic-index(rank()); }
	static constexpr size_t static_extent(rank_type) noexcept;
	constexpr index_type extent(rank_type) const noexcept;

	// [mdspan.extents.cons], constructors
	constexpr extents() noexcept = default;

	template<class _OtherIndexType, size_t... _OtherExtents>
	constexpr explicit(see below)
	extents(const extents<_OtherIndexType, _OtherExtents...>&) noexcept;
	template<class... _OtherIndexTypes>
	constexpr explicit extents(_OtherIndexTypes...) noexcept;
	template<class _OtherIndexType, size_t N>
	constexpr explicit(N != rank_dynamic())
	extents(span<_OtherIndexType, N>) noexcept;
	template<class _OtherIndexType, size_t N>
	constexpr explicit(N != rank_dynamic())
	extents(const array<_OtherIndexType, N>&) noexcept;

	// [mdspan.extents.cmp], comparison operators
	template<class _OtherIndexType, size_t... _OtherExtents>
	friend constexpr bool operator==(const extents&,
	const extents<_OtherIndexType, _OtherExtents...>&) noexcept;

	private:
	// libcxx note: we do not use an array here, but we need to preserve the as-if behavior
	// for example the default constructor must zero initialize dynamic extents
	array<index_type, rank_dynamic()> dynamic-extents{}; // exposition only
	};

	template<class... Integrals>
	explicit extents(Integrals...)
	-> see below;
	}

	// layout_left synopsis

	namespace std {
	template<class Extents>
	class layout_left::mapping {
	public:
	using extents_type = Extents;
	using index_type = typename extents_type::index_type;
	using size_type = typename extents_type::size_type;
	using rank_type = typename extents_type::rank_type;
	using layout_type = layout_left;

	// [mdspan.layout.right.cons], constructors
	constexpr mapping() noexcept = default;
	constexpr mapping(const mapping&) noexcept = default;
	constexpr mapping(const extents_type&) noexcept;
	template<class OtherExtents>
	constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
	mapping(const mapping<OtherExtents>&) noexcept;
	template<class OtherExtents>
	constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
	mapping(const layout_right::mapping<OtherExtents>&) noexcept;
	template<class OtherExtents>
	constexpr explicit(extents_type::rank() > 0)
	mapping(const layout_stride::mapping<OtherExtents>&) noexcept;

	constexpr mapping& operator=(const mapping&) noexcept = default;

	// [mdspan.layout.right.obs], observers
	constexpr const extents_type& extents() const noexcept { return extents_; }

	constexpr index_type required_span_size() const noexcept;

	template<class... Indices>
	constexpr index_type operator()(Indices...) const noexcept;

	static constexpr bool is_always_unique() noexcept { return true; }
	static constexpr bool is_always_exhaustive() noexcept { return true; }
	static constexpr bool is_always_strided() noexcept { return true; }

	static constexpr bool is_unique() noexcept { return true; }
	static constexpr bool is_exhaustive() noexcept { return true; }
	static constexpr bool is_strided() noexcept { return true; }

	constexpr index_type stride(rank_type) const noexcept;

	template<class OtherExtents>
	friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;

	private:
	extents_type extents_{}; // exposition only
	};
	}

	// layout_right synopsis

	namespace std {
	template<class Extents>
	class layout_right::mapping {
	public:
	using extents_type = Extents;
	using index_type = typename extents_type::index_type;
	using size_type = typename extents_type::size_type;
	using rank_type = typename extents_type::rank_type;
	using layout_type = layout_right;

	// [mdspan.layout.right.cons], constructors
	constexpr mapping() noexcept = default;
	constexpr mapping(const mapping&) noexcept = default;
	constexpr mapping(const extents_type&) noexcept;
	template<class OtherExtents>
	constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
	mapping(const mapping<OtherExtents>&) noexcept;
	template<class OtherExtents>
	constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
	mapping(const layout_left::mapping<OtherExtents>&) noexcept;
	template<class OtherExtents>
	constexpr explicit(extents_type::rank() > 0)
	mapping(const layout_stride::mapping<OtherExtents>&) noexcept;

	constexpr mapping& operator=(const mapping&) noexcept = default;

	// [mdspan.layout.right.obs], observers
	constexpr const extents_type& extents() const noexcept { return extents_; }

	constexpr index_type required_span_size() const noexcept;

	template<class... Indices>
	constexpr index_type operator()(Indices...) const noexcept;

	static constexpr bool is_always_unique() noexcept { return true; }
	static constexpr bool is_always_exhaustive() noexcept { return true; }
	static constexpr bool is_always_strided() noexcept { return true; }

	static constexpr bool is_unique() noexcept { return true; }
	static constexpr bool is_exhaustive() noexcept { return true; }
	static constexpr bool is_strided() noexcept { return true; }

	constexpr index_type stride(rank_type) const noexcept;

	template<class OtherExtents>
	friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;

	private:
	extents_type extents_{}; // exposition only
	};
	}

	// layout_stride synopsis

	namespace std {
	template<class Extents>
	class layout_stride::mapping {
	public:
	using extents_type = Extents;
	using index_type = typename extents_type::index_type;
	using size_type = typename extents_type::size_type;
	using rank_type = typename extents_type::rank_type;
	using layout_type = layout_stride;

	private:
	static constexpr rank_type rank_ = extents_type::rank(); // exposition only

	public:
	// [mdspan.layout.stride.cons], constructors
	constexpr mapping() noexcept;
	constexpr mapping(const mapping&) noexcept = default;
	template<class OtherIndexType>
	constexpr mapping(const extents_type&, span<OtherIndexType, rank_>) noexcept;
	template<class OtherIndexType>
	constexpr mapping(const extents_type&, const array<OtherIndexType, rank_>&) noexcept;

	template<class StridedLayoutMapping>
	constexpr explicit(see below) mapping(const StridedLayoutMapping&) noexcept;

	constexpr mapping& operator=(const mapping&) noexcept = default;

	// [mdspan.layout.stride.obs], observers
	constexpr const extents_type& extents() const noexcept { return extents_; }
	constexpr array<index_type, rank_> strides() const noexcept { return strides_; }

	constexpr index_type required_span_size() const noexcept;

	template<class... Indices>
	constexpr index_type operator()(Indices...) const noexcept;

	static constexpr bool is_always_unique() noexcept { return true; }
	static constexpr bool is_always_exhaustive() noexcept { return false; }
	static constexpr bool is_always_strided() noexcept { return true; }

	static constexpr bool is_unique() noexcept { return true; }
	constexpr bool is_exhaustive() const noexcept;
	static constexpr bool is_strided() noexcept { return true; }

	constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; }

	template<class OtherMapping>
	friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept;

	private:
	extents_type extents_{}; // exposition only
	array<index_type, rank_> strides_{}; // exposition only
	};
	}

	// default_accessor synopsis

	namespace std {
	template<class ElementType>
	struct default_accessor {
	using offset_policy = default_accessor;
	using element_type = ElementType;
	using reference = ElementType&;
	using data_handle_type = ElementType*;

	constexpr default_accessor() noexcept = default;
	template<class OtherElementType>
	constexpr default_accessor(default_accessor<OtherElementType>) noexcept;
	constexpr reference access(data_handle_type p, size_t i) const noexcept;
	constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
	};
	}

	// mdspan synopsis

	namespace std {
	template<class ElementType, class Extents, class LayoutPolicy = layout_right,
	class AccessorPolicy = default_accessor<ElementType>>
	class mdspan {
	public:
	using extents_type = Extents;
	using layout_type = LayoutPolicy;
	using accessor_type = AccessorPolicy;
	using mapping_type = typename layout_type::template mapping<extents_type>;
	using element_type = ElementType;
	using value_type = remove_cv_t<element_type>;
	using index_type = typename extents_type::index_type;
	using size_type = typename extents_type::size_type;
	using rank_type = typename extents_type::rank_type;
	using data_handle_type = typename accessor_type::data_handle_type;
	using reference = typename accessor_type::reference;

	static constexpr rank_type rank() noexcept { return extents_type::rank(); }
	static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
	static constexpr size_t static_extent(rank_type r) noexcept
	{ return extents_type::static_extent(r); }
	constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); }

	// [mdspan.mdspan.cons], constructors
	constexpr mdspan();
	constexpr mdspan(const mdspan& rhs) = default;
	constexpr mdspan(mdspan&& rhs) = default;

	template<class... OtherIndexTypes>
	constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts);
	template<class OtherIndexType, size_t N>
	constexpr explicit(N != rank_dynamic())
	mdspan(data_handle_type p, span<OtherIndexType, N> exts);
	template<class OtherIndexType, size_t N>
	constexpr explicit(N != rank_dynamic())
	mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);
	constexpr mdspan(data_handle_type p, const extents_type& ext);
	constexpr mdspan(data_handle_type p, const mapping_type& m);
	constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);

	template<class OtherElementType, class OtherExtents,
	class OtherLayoutPolicy, class OtherAccessorPolicy>
	constexpr explicit(see below)
	mdspan(const mdspan<OtherElementType, OtherExtents,
	OtherLayoutPolicy, OtherAccessorPolicy>& other);

	constexpr mdspan& operator=(const mdspan& rhs) = default;
	constexpr mdspan& operator=(mdspan&& rhs) = default;

	// [mdspan.mdspan.members], members
	template<class... OtherIndexTypes>
	constexpr reference operator[](OtherIndexTypes... indices) const;
	template<class OtherIndexType>
	constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
	template<class OtherIndexType>
	constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

	constexpr size_type size() const noexcept;
	[[nodiscard]] constexpr bool empty() const noexcept;

	friend constexpr void swap(mdspan& x, mdspan& y) noexcept;

	constexpr const extents_type& extents() const noexcept { return map_.extents(); }
	constexpr const data_handle_type& data_handle() const noexcept { return ptr_; }
	constexpr const mapping_type& mapping() const noexcept { return map_; }
	constexpr const accessor_type& accessor() const noexcept { return acc_; }

	// per LWG-4021 "mdspan::is_always_meow() should be noexcept"
	static constexpr bool is_always_unique() noexcept
	{ return mapping_type::is_always_unique(); }
	static constexpr bool is_always_exhaustive() noexcept
	{ return mapping_type::is_always_exhaustive(); }
	static constexpr bool is_always_strided() noexcept
	{ return mapping_type::is_always_strided(); }

	constexpr bool is_unique() const
	{ return map_.is_unique(); }
	constexpr bool is_exhaustive() const
	{ return map_.is_exhaustive(); }
	constexpr bool is_strided() const
	{ return map_.is_strided(); }
	constexpr index_type stride(rank_type r) const
	{ return map_.stride(r); }

	private:
	accessor_type acc_; // exposition only
	mapping_type map_; // exposition only
	data_handle_type ptr_; // exposition only
	};

	template<class CArray>
	requires(is_array_v<CArray> && rank_v<CArray> == 1)
	mdspan(CArray&)
	-> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;

	template<class Pointer>
	requires(is_pointer_v<remove_reference_t<Pointer>>)
	mdspan(Pointer&&)
	-> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;

	template<class ElementType, class... Integrals>
	requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
	explicit mdspan(ElementType*, Integrals...)
	-> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>; // until C++26
	template<class ElementType, class... Integrals>
	requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
	explicit mdspan(ElementType*, Integrals...)
	-> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>; // since C++26

	template<class ElementType, class OtherIndexType, size_t N>
	mdspan(ElementType*, span<OtherIndexType, N>)
	-> mdspan<ElementType, dextents<size_t, N>>;

	template<class ElementType, class OtherIndexType, size_t N>
	mdspan(ElementType*, const array<OtherIndexType, N>&)
	-> mdspan<ElementType, dextents<size_t, N>>;

	template<class ElementType, class IndexType, size_t... ExtentsPack>
	mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&)
	-> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;

	template<class ElementType, class MappingType>
	mdspan(ElementType*, const MappingType&)
	-> mdspan<ElementType, typename MappingType::extents_type,
	typename MappingType::layout_type>;

	template<class MappingType, class AccessorType>
	mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
	const AccessorType&)
	-> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
	typename MappingType::layout_type, AccessorType>;
	}
	*/

	#ifndef _LIBCPP_MDSPAN
	#define _LIBCPP_MDSPAN

	#include <__config>
	#include <__fwd/mdspan.h>
	#include <__mdspan/default_accessor.h>
	#include <__mdspan/extents.h>
	#include <__mdspan/layout_left.h>
	#include <__mdspan/layout_right.h>
	#include <__mdspan/layout_stride.h>
	#include <__mdspan/mdspan.h>
	#include <version>

	#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
	# pragma GCC system_header
	#endif

	#endif // _LIBCPP_MDSPAN