[PATCH v2 3/3] libstdc++: Adjust span/mdspan CTAD for P2781R9.
Luc Grosheintz
luc.grosheintz@gmail.com
Tue Sep 2 11:28:21 GMT 2025
On 9/2/25 10:21, Tomasz Kaminski wrote:
> On Tue, Sep 2, 2025 at 10:15 AM Luc Grosheintz <luc.grosheintz@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.grosheintz@gmail.com>
>> ---
>> libstdc++-v3/include/std/span | 3 ++-
>> .../23_containers/mdspan/extents/misc.cc | 20 +++++++++++++------
>> .../testsuite/23_containers/mdspan/mdspan.cc | 20 ++++++++++++-------
>> .../testsuite/23_containers/span/deduction.cc | 10 ++++++++++
>> 4 files changed, 39 insertions(+), 14 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)>>
>>
> On the last LWG telecon we have noticed, that this was not updated to
> remove_cvref_t above,
> and in consequence I believe, that:
> span(ptr, true_type{}); // does not work, it is not integral constant like
> span(ptr, cw<true>); // works,
>
> Could you add test like above, and let me know what are the result, so I
> can report back?
Let's look at:
std::span s16(a.data(), std::true_type{});
std::span s17(a.data(), std::cw<true>);
std::span s18(a.data(), true);
Quick reminder that `true_type` and `constant_wrapper` implicitly
convert to the underlying type, i.e. to bool.
1. Prior to any changes to the CTADs, s16 and s18, both compile and result
in dynamically sized spans:
https://godbolt.org/z/qeos99aMh
2. As per current wording of the standard and as implemented in v2 of this patch:
- s16 has dynamic extent.
- s17 has static extent `1`.
- s18 has dynamic extent.
3. As per your suggestion:
template<typename _Tp>
concept __integral_constant_like =
is_integral_v<remove_cvref_t<decltype(_Tp::value)>>
&& !is_same_v<bool, remove_cvref_t<decltype(_Tp::value)>>
&& convertible_to<_Tp, decltype(_Tp::value)>
&& equality_comparable_with<_Tp, decltype(_Tp::value)>
&& bool_constant<_Tp() == _Tp::value>::value
&& bool_constant<static_cast<decltype(_Tp::value)>(_Tp()) == _Tp::value>
::value;
All three cases are deduced as having a dynamic extent.
Could you please expand a little on what "works" means? Because the code
seems to quite explicitly not consider "wrapped booleans" as integer
constants, i.e.
&& !is_same_v<bool, remove_cvref_t<decltype(_Tp::value)>>
What's not clear to is if this is intentional, or if it's an attempt at
rejecting unrelated traits:
template<typename T>
class is_nice
{
constexpr static bool value = true;
};
(There's many `std::is_*` that have a static member `value` and I wouldn't
consider those "integer constants".)
Let me know if you need anything else.
> Do not send an v3 yet, until I will finish review.
>
>> && convertible_to<_Tp, decltype(_Tp::value)>
>> && equality_comparable_with<_Tp, decltype(_Tp::value)>
>> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> index 8a43a682004..018c4f95930 100644
>> --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> @@ -98,7 +98,7 @@ test_deduction(Extents... exts)
>> }
>>
>> constexpr bool
>> -test_integral_constant_deduction()
>> +test_deduction_from_constant()
>> {
>> auto verify = [](auto actual, auto expected)
>> {
>> @@ -106,13 +106,21 @@ test_integral_constant_deduction()
>> VERIFY(actual == expected);
>> };
>>
>> - constexpr auto c1 = std::integral_constant<size_t, 1>{};
>> - constexpr auto c2 = std::integral_constant<int, 2>{};
>> + constexpr auto i1 = std::integral_constant<size_t, 1>{};
>> + constexpr auto i2 = std::integral_constant<int, 2>{};
>>
>> verify(std::extents(1), std::extents<size_t, dyn>{1});
>> - verify(std::extents(c1), std::extents<size_t, 1>{});
>> + verify(std::extents(i1), std::extents<size_t, 1>{});
>> + verify(std::extents(i2), std::extents<size_t, 2>{});
>> + verify(std::extents(i1, 2), std::extents<size_t, 1, dyn>{2});
>> +
>> +#if __glibcxx_constant_wrapper
>> + constexpr auto c2 = std::constant_wrapper<2>{};
>> verify(std::extents(c2), std::extents<size_t, 2>{});
>> - verify(std::extents(c1, 2), std::extents<size_t, 1, dyn>{2});
>> + verify(std::extents(1, c2), std::extents<size_t, dyn, 2>{1});
>> + verify(std::extents(std::cw<2>), std::extents<size_t, 2>{});
>> + verify(std::extents(1, std::cw<2>), std::extents<size_t, dyn, 2>{1});
>> +#endif
>> return true;
>> }
>>
>> @@ -123,7 +131,7 @@ test_deduction_all()
>> test_deduction<1>(1);
>> test_deduction<2>(1.0, 2.0f);
>> test_deduction<3>(int(1), short(2), size_t(3));
>> - test_integral_constant_deduction();
>> + test_deduction_from_constant();
>> return true;
>> }
>>
>> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> index bdfc6ebcf56..2583047413c 100644
>> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> @@ -280,7 +280,7 @@ test_from_pointer_and_shape()
>> }
>>
>> constexpr bool
>> -test_from_pointer_and_integral_constant()
>> +test_from_pointer_and_constant()
>> {
>> std::array<double, 6> buffer{};
>> double * ptr = buffer.data();
>> @@ -292,12 +292,18 @@ test_from_pointer_and_integral_constant()
>> VERIFY(actual.extents() == expected.extents());
>> };
>>
>> - auto c3 = std::integral_constant<int, 3>{};
>> - auto c6 = std::integral_constant<int, 6>{};
>> + auto i3 = std::integral_constant<int, 3>{};
>> + auto i6 = std::integral_constant<int, 6>{};
>>
>> verify(std::mdspan(ptr, 6), std::extents(6));
>> - verify(std::mdspan(ptr, c6), std::extents(c6));
>> - verify(std::mdspan(ptr, 2, c3), std::extents(2, c3));
>> + verify(std::mdspan(ptr, i6), std::extents(i6));
>> + verify(std::mdspan(ptr, 2, i3), std::extents(2, i3));
>> +
>> +#if __glibcxx_constant_wrapper
>> + auto c3 = std::constant_wrapper<3>{};
>> + verify(std::mdspan(ptr, 2, c3), std::extents(2, i3));
>> + verify(std::mdspan(ptr, 2, std::cw<3>), std::extents(2, i3));
>> +#endif
>> return true;
>> }
>>
>> @@ -729,8 +735,8 @@ main()
>> test_from_pointer_and_shape();
>> static_assert(test_from_pointer_and_shape());
>>
>> - test_from_pointer_and_integral_constant();
>> - static_assert(test_from_pointer_and_integral_constant());
>> + test_from_pointer_and_constant();
>> + static_assert(test_from_pointer_and_constant());
>>
>> test_from_extents();
>> static_assert(test_from_extents());
>> diff --git a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> index c66db90222e..0d045505bed 100644
>> --- a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> +++ b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> @@ -83,4 +83,14 @@ test01()
>>
>> std::span s13(a.data(), std::integral_constant<size_t, 3>{});
>> static_assert( is_static_span<long, 3>(s13));
>> +
>> +#if __glibcxx_constant_wrapper
>> + auto c5 = std::constant_wrapper<5>{};
>> +
>> + std::span s14(a.data(), std::cw<4>);
>> + static_assert( is_static_span<long, 4>(s14));
>> +
>> + std::span s15(a.data(), c5);
>> + static_assert( is_static_span<long, 5>(s15));
>> +#endif
>> }
>> --
>> 2.51.0
>>
>>
>
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