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.
