Pushed to original patch (without LWG4351 changes) to trunk.
On Sat, Sep 6, 2025 at 3:18 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> On 9/5/25 19:58, Jonathan Wakely wrote:
> > On Fri, 5 Sept 2025 at 16:26, Jonathan Wakely <jwak...@redhat.com>
> wrote:
> >>
> >> On Thu, 4 Sept 2025 at 13:22, Luc Grosheintz <luc.groshei...@gmail.com>
> wrote:
> >>>
> >>> A usecase for P2781R9 is more ergonomic creation of span and mdspan
> with
> >>> mixed static and dynamic extents, e.g.:
> >>>
> >>> span(ptr, cw<3>)
> >>> extents(cw<3>, 5, cw<7>)
> >>> mdspan(ptr, cw<3>, 5, cw<7>)
> >>>
> >>> should be deduced as:
> >>> span<..., 3>
> >>> extents<..., 3, dyn, 7>
> >>> mdspan<..., extents<..., 3, dyn, 7>>
> >>>
> >>> The change required is to strip cv-qualifiers and references from
> >>> `_Tp::value`, because of:
> >>>
> >>> template<_CwFixedValue _X, typename>
> >>> struct constant_wrapper : _CwOperators
> >>> {
> >>> static constexpr const auto& value = _X._M_data;
> >>>
> >>> libstdc++-v3/ChangeLog:
> >>>
> >>> * include/std/span (__integral_constant_like): Allow the
> member
> >>> `value` of a constant wrapping type to be a const reference of
> >>> an integer.
> >>> * testsuite/23_containers/mdspan/extents/misc.cc: Add test for
> >>> cw and constant_wrapper.
> >>> * testsuite/23_containers/mdspan/mdspan.cc: Ditto.
> >>> * testsuite/23_containers/span/deduction.cc: Ditto.
> >>>
> >>> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >>> ---
> >>> libstdc++-v3/include/std/span | 3 ++-
> >>> .../23_containers/mdspan/extents/misc.cc | 22 +++++++++++++-----
> >>> .../testsuite/23_containers/mdspan/mdspan.cc | 23
> +++++++++++++------
> >>> .../testsuite/23_containers/span/deduction.cc | 20 +++++++++++++++-
> >>> 4 files changed, 53 insertions(+), 15 deletions(-)
> >>>
> >>> diff --git a/libstdc++-v3/include/std/span
> b/libstdc++-v3/include/std/span
> >>> index 44f9b36a7ef..f9aa3c77e8e 100644
> >>> --- a/libstdc++-v3/include/std/span
> >>> +++ b/libstdc++-v3/include/std/span
> >>> @@ -480,7 +480,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>> namespace __detail
> >>> {
> >>> template<typename _Tp>
> >>> - concept __integral_constant_like =
> is_integral_v<decltype(_Tp::value)>
> >>> + concept __integral_constant_like =
> >>> + is_integral_v<remove_cvref_t<decltype(_Tp::value)>>
> >>> && !is_same_v<bool, remove_const_t<decltype(_Tp::value)>>
> >>
> >> We also need remove_cvref_t on this line for the not-bool check, so
> >> that constant_wrapper<bool&> doesn't match here, but I haven't opened
> >> an LWG issue about that yet, so we can fix that later.
> >
> > I've created https://cplusplus.github.io/LWG/issue4351 now.
> >
>
> Here's a patch to apply the change:
> https://gcc.gnu.org/pipermail/libstdc++/2025-September/063315.html
>
> if you prefer to squash it into this commit, let me know.
>
>
