Proposed resolution:

This wording is relative to N5014.

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

   -5- Mandates:

   1. (5.1) — […]

   2. (5.2) — if `padding_value` is not equal to `dynamic_extent`, then `padding_value` is representable as a value of type `index_type`.

   3. (5.3) — […]

   4. (5.4) — […]

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

   -5- Mandates:

   1. (5.1) — […]

   2. (5.2) — if `padding_value` is not equal to `dynamic_extent`, then `padding_value` is representable as a value of type `index_type`.

   3. (5.3) — […]

   4. (5.4) — […]

Two new layouts were added to <mdspan> in C++26. Both have a template parameter `size_t PaddingValue`. This value is allowed to be `std::dynamic_extent` to signal that the padding value isn't known at compile time.

A class Mandates: element (in [mdspan.layout.leftpad.overview] (5.2) and [mdspan.layout.rightpad.overview] (5.2), respectively) requires (unconditionally) that

• `PaddingValue` is representable as a value of `index_type`.

Since `std::dynamic_extent` is defined as `size_t(-1)` (in [span.syn]) this immediately prohibits all dynamically padded layout mappings for any `index_type` for which:

numeric_limit<index_type>::max() < numeric_limit<size_t>::max()

One example is `int` on a 64-bit system.

The proposed resolution states that the modified representability Mandates: element holds for rank <= 1, even though in that case the `PaddingValue` has no other effect. Hence, the Mandates: element could be weakened further.