libstdc++: potential bug in specification of layouts.

[Luc Grosheintz]

This email chain continues a discussion about potential bugs in the
specification that was started here:
https://gcc.gnu.org/pipermail/libstdc++/2025-May/061350.html

For notational convenience, I'll write `layout_left` as shorthand for
`layout_left::mapping`, e.g. layout_left::required_span_size, refers to
layout_left::mapping<Extents>::required_span_size.


1. operator==

The observation is that for rank0 and rank1, two mappings, one of type
layout_left<Extents> and the other layout_right<OExtents>, the
operator== is ambiguous if `Extents == OExtents`; but not if the extents
are convertible (because the convertability is only in one direction).

Godbolt: https://godbolt.org/z/35TbMnc1e

Resolution: It's not clear to me what to propose. Likely, something that
fixes the ambiguity issue to allow comparing all rank 0 and rank 1
instances of layout_left and layout_right. Help would be most welcome:
either by collaboratively working on the proposed change; or if someone
wants to take over completely that would also work.


2. Missing `noexcept` in layout_left.

The specification (N4950 and as published on `eel.is`) consistently
doesn't mention `noexcept` in both the Overview and Constructors
sections for `layout_left(layout_stride)`.

https://eel.is/c++draft/mdspan.layout.left#overview-1
https://eel.is/c++draft/mdspan.layout.left#lib:layout_left::mapping,constructor____

Resolution: I suggest that I file an issue to add `noexcept`
consistently. Can this be considered an editorial issue?


3. Inconsistency about `constexpr` in layout_right::required_span_size.

The section Overview specifies that `required_span_size` must be
constexpr. In the section Observers this keyword is missing.

I believe this is considered an editorial issue; and I can fix it by
submitting a PR to the text of the standard.

https://eel.is/c++draft/mdspan.layout.right#overview-1
https://eel.is/c++draft/mdspan.layout.right.obs#lib:required_span_size,layout_right::mapping

Resolution: I suggest that I file an editorial issue.


4. Inconsistency about `noexcept` in `layout_left::stride`.

The section Overview mentions it's `noexcept` but the section Observers
doesn't. Again, I think this is an editorial issue.

https://eel.is/c++draft/mdspan.layout.left#overview-1
https://eel.is/c++draft/mdspan.layout.left.obs#lib:stride,layout_left::mapping

Resolution: I suggest that I file an editorial issue.


5. At Rank 0 layout_stride is untypically convertible.

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

https://eel.is/c++draft/mdspan.layout.left.cons#lib:layout_left::mapping,constructor_
https://eel.is/c++draft/mdspan.layout.left.cons#lib:layout_left::mapping,constructor__
https://eel.is/c++draft/mdspan.layout.right.cons#lib:layout_right::mapping,constructor_
https://eel.is/c++draft/mdspan.layout.right.cons#lib:layout_right::mapping,constructor__
https://eel.is/c++draft/mdspan.layout.stride.cons#9

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>);

https://eel.is/c++draft/mdspan.layout.left.cons#lib:layout_left::mapping,constructor____
https://eel.is/c++draft/mdspan.layout.right.cons#lib:layout_right::mapping,constructor____

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`.

Godbolt: https://godbolt.org/z/MdbvafG7n

Resolution: I suggest I create an non-editorial issue to propose adding
the convertibility criterium to the explicitness condition.