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4272. For rank == 0, layout_stride is atypically convertible

Section: 23.7.3.4 [mdspan.layout] Status: New Submitter: Luc Grosheintz Opened: 2025-06-02 Last modified: 2025-06-12

Priority: 2

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Discussion:

Commonly, two layouts are considered convertible, if the underlying extent_types are convertible.

However, for the two ctors layout_left(layout_stride) and layout_right(layout_stride), the condition is rank > 0. Therefore,

using E1 = std::extents<int>;
using E2 = std::extents<unsigned int>;

static_assert(std::is_convertible_v<
    std::layout_stride::mapping<E2>,
    std::layout_right::mapping<E1>
  >);

even though:

static_assert(!std::is_convertible_v<E2, E1>);

Moreover, for rank 0 layout_stride can be converted to any specialization of layout_left or layout_right; but not to every specialization of layout_stride.

[2025-06-12; Reflector poll]

Set priority to 2 after reflector poll.

Proposed resolution:

This wording is relative to N5008.

[Drafting note: As drive-by fixes the edits for layout_left_padded<>::mapping and layout_right_padded<>::mapping also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct spelling of the exposition-only data member rank_.]

1. Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:

   namespace std {
     template<class Extents>
     class layout_left::mapping {
       […]
       // 23.7.3.4.5.2 [mdspan.layout.left.cons], constructors
       […]
       template<class OtherExtents>
         constexpr explicit(extents_type::rank() > 0see below)
           mapping(const layout_stride::mapping<OtherExtents>&);
       
       constexpr mapping& operator=(const mapping&) noexcept = default;
       […]
     };
   }
   

2. Modify 23.7.3.4.5.2 [mdspan.layout.left.cons] as indicated:

   template<class OtherExtents>
     constexpr explicit(extents_type::rank() > 0see below)
       mapping(const layout_stride::mapping<OtherExtents>& other);
   

   -13- Constraints: […]

   -14- Preconditions: […]

   -15- Effects: […]

   -?- Remarks: The expression inside explicit is equivalent to:

   !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)
   

3. Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:

   namespace std {
     template<class Extents>
     class layout_right::mapping {
       […]
       // 23.7.3.4.6.2 [mdspan.layout.right.cons], constructors
       […]
       template<class OtherExtents>
         constexpr explicit(extents_type::rank() > 0see below)
           mapping(const layout_stride::mapping<OtherExtents>&);
       
       constexpr mapping& operator=(const mapping&) noexcept = default;
       […]
     };
   }
   

4. Modify 23.7.3.4.6.2 [mdspan.layout.right.cons] as indicated:

   template<class OtherExtents>
     constexpr explicit(extents_type::rank() > 0see below)
       mapping(const layout_stride::mapping<OtherExtents>& other);
   

   -13- Constraints: […]

   -14- Preconditions: […]

   -15- Effects: […]

   -?- Remarks: The expression inside explicit is equivalent to:

   !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)
   

5. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   namespace std {
     template<size_t PaddingValue>
     template<class Extents>
     class layout_left_padded<PaddingValue>::mapping {
       […]
       // 23.7.3.4.8.3 [mdspan.layout.leftpad.cons], constructors
       […]
       template<class OtherExtents>
         constexpr explicit(extents_type::rank() > 0see below)
           mapping(const layout_stride::mapping<OtherExtents>&);
       […]
     };
   }
   

6. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   template<class OtherExtents>
     constexpr explicit(rank_ > 0see below)
       mapping(const layout_stride::mapping<OtherExtents>& other);
   

   -10- Constraints: […]

   -11- Preconditions: […]

   -12- Effects: […]

   -?- Remarks: The expression inside explicit is equivalent to:

   !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)
   

7. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   namespace std {
     template<size_t PaddingValue>
     template<class Extents>
     class layout_right_padded<PaddingValue>::mapping {
       […]
       // 23.7.3.4.9.3 [mdspan.layout.rightpad.cons], constructors
       […]
       template<class OtherExtents>
         constexpr explicit(rank_ > 0see below)
           mapping(const layout_stride::mapping<OtherExtents>&);
       […]
     };
   }
   

8. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   template<class OtherExtents>
     constexpr explicit(rank_ > 0see below)
       mapping(const layout_stride::mapping<OtherExtents>& other);
   

   -10- Constraints: […]

   -11- Preconditions: […]

   -12- Effects: […]

   -?- Remarks: The expression inside explicit is equivalent to:

   !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)