libstdc++: potential bug in specification of layouts.

[Tomasz Kaminski]

On Tue, May 13, 2025 at 11:55 AM Luc Grosheintz <luc.grosheintz@gmail.com>
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.
>
I think that this could be fixed by changing the friend operator== in
layout_stride,
to generic overload in std:
template<typename StridedMapping1, typename StridedMapping2>
constexpr bool operator==(const StridedMapping1& lhs, const
StridedMapping2& rhs);
Constrains:
  ** **layout-mapping-alike*
<https://en.cppreference.com/w/cpp/named_req/LayoutMapping#layout-mapping-alike>
 <StridedMapping1> && *layout-mapping-alike*
<https://en.cppreference.com/w/cpp/named_req/LayoutMapping#layout-mapping-alike>
 <StridedMapping12>
  * StridedMapping1::extents_type::rank() ==
StridedMapping2::extents_type::rank()
  * StridedMapping1::is_always_strided() && StridedMapping2::
is_always_strided()
This would allow any strided mapping to be compared without conversions.


>
> 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.
>
>
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