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4405. mdspan constructor should disallow derived to base conversions

Section: 23.7.3.6.2 [mdspan.mdspan.cons] Status: New Submitter: Hewill Kang Opened: 2025-10-05 Last modified: 2025-10-10

Priority: Not Prioritized

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Discussion:

Unlike ranges::subrange or span, mdspan syntactically allows a multidimensional viewing base class via a derived class pointer (demo):

#include <span>
#include <ranges>
#include <mdspan>

struct Base {};
struct Derived : Base {};
std::array<Derived, 12> arr;
std::ranges::subrange<Base*> s(arr); // error, slicing
std::span<Base> sp(arr.data(), arr.size()); // error, slicing
std::mdspan<Base, std::dims<1>> msp(arr.data(), arr.size()); // ok

Given that we intend to reject object slicing for both default_accessor and aligned_accessor, there seems no reason not to reject this invalid pointer arithmetic for mdspan.

Proposed resolution:

This wording is relative to N5014.

[Drafting note: The exposition-only concept convertible-to-non-slicing comes from 25.5.4.1 [range.subrange.general].]

1. Modify 23.7.3.6.1 [mdspan.mdspan.overview] as indicated:

   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;
       […]
       template<class... OtherIndexTypes>
         constexpr explicit mdspan(convertible-to-non-slicing<data_handle_type> auto ptr, OtherIndexTypes... exts);
       template<class OtherIndexType, size_t N>
         constexpr explicit(N != rank_dynamic())
           mdspan(convertible-to-non-slicing<data_handle_type> auto p, span<OtherIndexType, N> exts);
       template<class OtherIndexType, size_t N>
         constexpr explicit(N != rank_dynamic())
           mdspan(convertible-to-non-slicing<data_handle_type> auto p, const array<OtherIndexType, N>& exts);
       constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const extents_type& ext);
       constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const mapping_type& m);
       constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const mapping_type& m, const accessor_type& a);
       […]
     };
     […]
   }
   

2. Modify 23.7.3.6.2 [mdspan.mdspan.cons] as indicated:

   template<class... OtherIndexTypes>
     constexpr explicit mdspan(convertible-to-non-slicing<data_handle_type> auto p, OtherIndexTypes... exts);
   

   -4- Let N be sizeof...(OtherIndexTypes).

   -5- Constraints: […]

   […]

   template<class OtherIndexType, size_t N>
     constexpr explicit(N != rank_dynamic())
       mdspan(convertible-to-non-slicing<data_handle_type> auto p, span<OtherIndexType, N> exts);
   template<class OtherIndexType, size_t N>
     constexpr explicit(N != rank_dynamic())
       mdspan(convertible-to-non-slicing<data_handle_type> auto p, const array<OtherIndexType, N>& exts);
   

   -8- Constraints: […]

   […]

   constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const extents_type& ext);
   

   -11- Constraints: […]

   […]

   constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const mapping_type& m);
   

   -14- Constraints: […]

   […]

   constexpr mdspan(convertible-to-non-slicing<data_handle_type> auto p, const mapping_type& m, const accessor_type& a);
   

   -17- Preconditions: […]

   […]