Proposed resolution:

This wording is relative to N5032.

1. Modify [mdspan.layout.leftpad.cons] as indicated:

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

   -10- Constraints: […]

   -11- Preconditions:

   • (11.1) — […]

   • (11.2) — […]

   • (11.3) — […]

   • (11.4) — `other.required_span_size()` is representable as a value of type index_type.

   • (11.5) — If rank_ is greater than `1` and static-padding-stride equals `dynamic_extent`, `other.stride(1)` is representable as a value of type index_type.

   -12- Effects: […]

   -13- Remarks: […]

   template<class LayoutLeftPaddedMapping>
     constexpr explicit(see below)
       mapping(const LayoutLeftPaddedMapping& other);
   

   -14- Constraints: […]

   -15- Mandates: […]

   -16- Preconditions:

   • (16.1) — […]

   • (16.2) — `other.required_span_size()` is representable as a value of type index_type.

   • (16.3) — If rank_ is greater than `1` and static-padding-stride equals `dynamic_extent`, `other.stride(1)` is representable as a value of type index_type.

   -17- Effects: […]

   -18- Remarks: […]

2. Modify [mdspan.layout.rightpad.cons] as indicated:

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

   -10- Constraints: […]

   -11- Preconditions:

   • (11.1) — […]

   • (11.2) — […]

   • (11.3) — […]

   • (11.4) — `other.required_span_size()` is representable as a value of type index_type.

   • (11.5) — If rank_ is greater than `1` and static-padding-stride equals `dynamic_extent`, other.stride(rank_ - 2) is representable as a value of type index_type.

   -12- Effects: […]

   -13- Remarks: […]

   template<class LayoutRightPaddedMapping>
     constexpr explicit(see below)
       mapping(const LayoutRightPaddedMapping& other);
   

   -14- Constraints: […]

   -15- Mandates: […]

   -16- Preconditions:

   • (16.1) — […]

   • (16.2) — `other.required_span_size()` is representable as a value of type index_type.

   • (16.3) — If rank_ is greater than `1` and static-padding-stride equals `dynamic_extent`, other.stride(rank_ - 2) is representable as a value of type index_type.

   -17- Effects: […]

   -18- Remarks: […]

It appears that these constructors are missing a precondition requiring the incoming padded stride to be representable as a value of the destination `index_type`.

The existing preconditions check that `other.required_span_size()` is representable as a value of type `index_type`, but that is not sufficient when the source mapping is empty. In that case, `other.required_span_size()` can be `0` even though the source padded stride is too large for the destination `index_type`. The padded stride is then silently truncated, and that truncation is observable through `stride()`.

Example:

#include <mdspan>
#include <cstdint>
#include <cassert>

void test() {
  using Src = std::layout_left_padded<std::dynamic_extent>::mapping<std::extents<uint32_t, 1, 0>>;
  using Dst = std::layout_left_padded<std::dynamic_extent>::mapping<std::extents<uint8_t, 1, 0>>;

  // src.stride(1) == 256.
  Src src(std::extents<uint32_t, 1, 0>(), 256);

  // other.required_span_size() is 0, so the existing preconditions are met.
  // The stored padded stride is then converted to uint8_t and truncated to 0.
  Dst dst(src);

  assert(dst.stride(1) == 0);
}