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4266. layout_stride::mapping should treat empty mappings as exhaustive

Section: 23.7.3.4.7 [mdspan.layout.stride] Status: New Submitter: Tomasz Kamiński Opened: 2025-05-22 Last modified: 2025-05-24

Priority: Not Prioritized

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Discussion:

Mapping over an empty multidimensional index space is always exhaustive according to the corresponding definitions from 23.7.3.4.2 [mdspan.layout.reqmts] p16.

However, the current specification of layout_stride::mapping does not consider whether some of the empty multidimensional index spaces are unique or exhaustive. For illustration, the mapping with the following configuration is not considered exhaustive according to the current specification of 23.7.3.4.7.4 [mdspan.layout.stride.obs] bullet 5.2:

extents: 2, 2, 0
strides: 2, 6, 20

This prevents the implementation from implementing sm.is_exhaustive() as sm.fwd-prod-of-extents(sm::extents_type::rank()) == sm.required_span_size(). For all mappings with size greater than zero, such an expression provides an answer consistent with the standard. However, it always returns true for an empty mapping, such as shown in the example.

We should make such implementation conforming, and require is_exhaustive() to return true for empty mappings.

For consistency, we could update is_always_exhaustive() to recognize mapping with rank() == 0, and one for which at least one of the static extents is equal to zero (i.e., they always represent a multidimensional index space).

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.3.4.7.1 [mdspan.layout.stride.overview] as indicated:

   namespace std {
     template<class Extents>
     class layout_stride::mapping {
       […]
       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; }
       […]
     };
   }
   

2. Modify 23.7.3.4.7.4 [mdspan.layout.stride.obs] as indicated:

   […]

   static constexpr bool is_always_exhaustive() noexcept;
   

   -?- Returns: true if rank_ is 0 or if there is a rank index r of extents() such that extents_type::static_extent(r) is 0, otherwise false.

   constexpr bool is_exhaustive() const noexcept;
   

   -5- Returns:

   1. (5.1) — true if rank_ or the size of the multidimensional index space m.extents() is 0.

   2. (5.2) — […]

   3. (5.3) — […]