libstdc++: potential bug in specification of layouts.

[Luc Grosheintz]

I have a seventh question; and I think keeping them all together
might be convenient.

On 5/13/25 11:51, Luc Grosheintz wrote:
> 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.

7. Missing Prerequisite representable as size_t.

The mapping policy requires that m(i...) is representable as both index_type
and size_t.

https://eel.is/c++draft/mdspan.layout#reqmts-8

However, I can only find a prerequisite that requires the extents to be such
that the required_span_size is representable as index_type. I can't find
anything that ensures representable as size_t, e.g.

https://eel.is/c++draft/mdspan.layout#left.cons-1

Is this a missing prerequisite?

My initial understanding (possibly faulty) was that the layouts did all
their computations in `index_type` and that it would allow computing
mappings even if size_t was too small, e.g. when dealing with 32-bit
systems and either disks or distributed systems. In both cases a multi-
dimensional array could be much larger than the addressable RAM; and
while that would require some care and mdspan might/can't not work, one
might still want to compute the mapping of multi-dimensional indexes to
flat index. Moreover, the layouts don't introduce any notion of underlying
memory (other than the requirement that m(i...) is representable as
size_t).