libstdc++: potential bug in specification of layouts.

Tue May 13 11:23:21 GMT 2025

On Tue, May 13, 2025 at 12:55 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Tue, 13 May 2025 at 10:54, 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.
>
> Thanks for writing these up, Luc.
>
> Once the discussion concludes here you can submit one or more issues
> to the LWG chair (which is, er, me) as documented at
> https://cplusplus.github.io/LWG/lwg-active.html#submit_issue
>
>
> > 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.
>
> N.B. it's not necessary to have a resolution when reporting an issue.
> Generally issues will get resolved sooner if there's a proposed
> resolution (even if it's a bad one, because that might nerdsnipe
> somebody into writing a better one!
> https://meta.wikimedia.org/wiki/Cunningham%27s_Law applies here).
>
>
> >
> >
> > 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?
>
> Since it's *consistently* missing, I don't think we can consider it
> editorial. What is in the standard matches what was approved by the
> committee.
>
It is missing for layout_left, however the corresponding constructor for
layout_right has noexcept
specifier, despite the same argument applying to it.
layout_left_padded, layout_right_padded also do not have noexcept here, so
maye layout_right should
be adjusted, but I believe these constructors should be consistent.

>
> And this seems to be an intentional application of the "Lakos rule"
> which says that functions with narrow contracts (i.e. with
> preconditions) should not be noexcept. We can strengthen that and add
> noexcept in the implementation, if we know that our implementation
> never throws exceptions as a result of precondition violations (and
> will never do so in future, e.g. if we were to start using C++26
> contracts to check preconditions, which would mean the constructors
> could throw if the user provides a throwing contract violation
> handler).
>
>
> >
> > 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.
>
> I see you already filed that, thanks.
>
> >
> > 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.
>
> That was recently fixed editorially:
> https://github.com/cplusplus/draft/pull/7831
> The unofficial HTML draft is out of date.
>
> >
> >
> > 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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