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.
