This was merged. Thanks for working on it.
However, the constant_wrapper paper accepted in Sofia, modifies the concept:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2781r8.html
On Tue, Jul 8, 2025 at 1:01 PM Tomasz Kaminski <tkami...@redhat.com> wrote:
> Maybe adding a negative test for the error when converting negative values
> would be valuable then.
> I think we can add it as separate commit later, I will try to get
> everything merged today,
>
> On Tue, Jul 8, 2025 at 12:56 PM Jonathan Wakely <jwak...@redhat.com>
> wrote:
>
>> On Tue, 8 Jul 2025 at 11:46, Tomasz Kaminski <tkami...@redhat.com> wrote:
>> >
>> >
>> >
>> > On Tue, Jul 8, 2025 at 12:08 PM Jonathan Wakely <jwak...@redhat.com>
>> wrote:
>> >>
>> >> On Tue, 8 Jul 2025 at 10:54, Luc Grosheintz <luc.groshei...@gmail.com>
>> wrote:
>> >> >
>> >> > This implements P3029R1. In P3029R1, the CTAD for span is refined to
>> >> > permit deducing the extent of the span from an integral constant,
>> e.g.
>> >> >
>> >> > span((T*) ptr, integral_constant<size_t, 5>{});
>> >> >
>> >> > is deduced as span<T, 5>. Similarly, in
>> >> >
>> >> > auto exts = extents(integral_constant<int, 2>);
>> >> > auto md = mdspan((T*) ptr, integral_constant<int, 2>);
>> >> >
>> >> > exts and md have types extents<size_t, 2> and mdspan<double,
>> >> > extents<size_t, 2>>, respectively.
>> >> >
>> >> > PR libstdc++/120914
>> >> >
>> >> > libstdc++-v3/ChangeLog:
>> >> >
>> >> > * include/std/span (span): Update CTAD to enable
>> >> > integral constants [P3029R1].
>> >> > * include/std/mdspan (extents): ditto.
>> >> > (mdspan): ditto.
>> >> > * testsuite/23_containers/span/deduction.cc: Test deduction
>> >> > guide.
>> >> > * testsuite/23_containers/mdspan/extents/misc.cc: ditto.
>> >> > * testsuite/23_containers/mdspan/mdspan.cc: ditto.
>> >> >
>> >> > Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
>> >> > ---
>> >> > libstdc++-v3/include/std/mdspan | 8 ++----
>> >> > libstdc++-v3/include/std/span | 20 ++++++++++++++-
>> >> > .../23_containers/mdspan/extents/misc.cc | 20 +++++++++++++++
>> >> > .../testsuite/23_containers/mdspan/mdspan.cc | 25
>> +++++++++++++++++++
>> >> > .../testsuite/23_containers/span/deduction.cc | 3 +++
>> >> > 5 files changed, 69 insertions(+), 7 deletions(-)
>> >> >
>> >> > diff --git a/libstdc++-v3/include/std/mdspan
>> b/libstdc++-v3/include/std/mdspan
>> >> > index 0fd78570b3e..152dcb3e92a 100644
>> >> > --- a/libstdc++-v3/include/std/mdspan
>> >> > +++ b/libstdc++-v3/include/std/mdspan
>> >> > @@ -406,10 +406,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >> > template<typename _IndexType, size_t... _Counts>
>> >> > auto __build_dextents_type(integer_sequence<size_t,
>> _Counts...>)
>> >> > -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
>> >> > -
>> >> > - template<typename _Tp>
>> >> > - consteval size_t
>> >> > - __dynamic_extent() { return dynamic_extent; }
>> >> > }
>> >> >
>> >> > template<typename _IndexType, size_t _Rank>
>> >> > @@ -419,7 +415,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >> > template<typename... _Integrals>
>> >> > requires (is_convertible_v<_Integrals, size_t> && ...)
>> >> > explicit extents(_Integrals...) ->
>> >> > - extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>;
>> >> > + extents<size_t, __detail::__maybe_static_ext<_Integrals>...>;
>> >> >
>> >> > struct layout_left
>> >> > {
>> >> > @@ -1316,7 +1312,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >> > && (sizeof...(_Integrals) > 0)
>> >> > explicit mdspan(_ElementType*, _Integrals...)
>> >> > -> mdspan<_ElementType,
>> >> > - extents<size_t,
>> __mdspan::__dynamic_extent<_Integrals>()...>>;
>> >> > + extents<size_t,
>> __detail::__maybe_static_ext<_Integrals>...>>;
>> >> >
>> >> > template<typename _ElementType, typename _OIndexType, size_t _Nm>
>> >> > mdspan(_ElementType*, span<_OIndexType, _Nm>)
>> >> > diff --git a/libstdc++-v3/include/std/span
>> b/libstdc++-v3/include/std/span
>> >> > index 49ab9109d83..5629a71b9bd 100644
>> >> > --- a/libstdc++-v3/include/std/span
>> >> > +++ b/libstdc++-v3/include/std/span
>> >> > @@ -476,6 +476,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >> > };
>> >> >
>> >> > // deduction guides
>> >> > + namespace __detail
>> >> > + {
>> >> > + template<typename _Tp>
>> >> > + concept __integral_constant_like =
>> is_integral_v<decltype(_Tp::value)>
>> >> > + && !is_same_v<bool, remove_const_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;
>> >> > +
>> >> > + template<typename _Tp>
>> >> > + constexpr size_t __maybe_static_ext = dynamic_extent;
>> >> > +
>> >> > + template<__integral_constant_like _Tp>
>> >> > + constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
>> >>
>> >> Are the braces here to detect narrowing conversions?
>> >>
>> >> (The paper doesn't mention why they're used, as far as I can see)
>> >
>> > The information is in revision history (a bit hidden):
>> > Initialized the maybe-static-ext's specialization with {T::value} to
>> prevent conversions from negative values.
>>
>> Thanks. OK for trunk then.
>>
>>
>> >>
>> >>
>> >>
>> >> > + }
>> >> >
>> >> > template<typename _Type, size_t _ArrayExtent>
>> >> > span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
>> >> > @@ -489,7 +506,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >> >
>> >> > template<contiguous_iterator _Iter, typename _End>
>> >> > span(_Iter, _End)
>> >> > - -> span<remove_reference_t<iter_reference_t<_Iter>>>;
>> >> > + -> span<remove_reference_t<iter_reference_t<_Iter>>,
>> >> > + __detail::__maybe_static_ext<_End>>;
>> >> >
>> >> > template<ranges::contiguous_range _Range>
>> >> > span(_Range &&)
>> >> > diff --git
>> a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> >> > index e71fdc54230..bca8901685d 100644
>> >> > --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> >> > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
>> >> > @@ -97,6 +97,25 @@ test_deduction(Extents... exts)
>> >> > VERIFY(e == expected);
>> >> > }
>> >> >
>> >> > +constexpr bool
>> >> > +test_integral_constant_deduction()
>> >> > +{
>> >> > + auto verify = [](auto actual, auto expected)
>> >> > + {
>> >> > + static_assert(std::same_as<decltype(actual),
>> decltype(expected)>);
>> >> > + VERIFY(actual == expected);
>> >> > + };
>> >> > +
>> >> > + constexpr auto c1 = std::integral_constant<size_t, 1>{};
>> >> > + constexpr auto c2 = 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(c2), std::extents<size_t, 2>{});
>> >> > + verify(std::extents(c1, 2), std::extents<size_t, 1, dyn>{2});
>> >> > + return true;
>> >> > +}
>> >> > +
>> >> > constexpr bool
>> >> > test_deduction_all()
>> >> > {
>> >> > @@ -104,6 +123,7 @@ 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();
>> >> > return true;
>> >> > }
>> >> >
>> >> > diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> >> > index 9252273bf66..a650fb19bdf 100644
>> >> > --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> >> > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
>> >> > @@ -245,6 +245,28 @@ test_from_pointer_and_shape()
>> >> > return true;
>> >> > }
>> >> >
>> >> > +constexpr bool
>> >> > +test_from_pointer_and_integral_constant()
>> >> > +{
>> >> > + std::array<double, 6> buffer{};
>> >> > + double * ptr = buffer.data();
>> >> > +
>> >> > + auto verify = [ptr](auto actual, auto exts)
>> >> > + {
>> >> > + auto expected = std::mdspan<double, decltype(exts)>(ptr,
>> exts);
>> >> > + static_assert(std::same_as<decltype(actual),
>> decltype(expected)>);
>> >> > + VERIFY(actual.extents() == expected.extents());
>> >> > + };
>> >> > +
>> >> > + auto c3 = std::integral_constant<int, 3>{};
>> >> > + auto c6 = 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));
>> >> > + return true;
>> >> > +}
>> >> > +
>> >> > constexpr bool
>> >> > test_from_extents()
>> >> > {
>> >> > @@ -616,6 +638,9 @@ 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_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 dce6cedf89b..c66db90222e 100644
>> >> > --- a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> >> > +++ b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
>> >> > @@ -80,4 +80,7 @@ test01()
>> >> >
>> >> > std::span s12(const_cast<const std::span<int>&>(s5));
>> >> > static_assert( is_dynamic_span<int>(s12) );
>> >> > +
>> >> > + std::span s13(a.data(), std::integral_constant<size_t, 3>{});
>> >> > + static_assert( is_static_span<long, 3>(s13));
>> >> > }
>> >> > --
>> >> > 2.49.0
>> >> >
>> >>
>>
>>
