https://gcc.gnu.org/g:9c600a7e6cc588d2ee79d764cbf69ad677b1bac5
commit r16-2106-g9c600a7e6cc588d2ee79d764cbf69ad677b1bac5
Author: Luc Grosheintz <luc.groshei...@gmail.com>
Date: Tue Jul 8 11:49:21 2025 +0200
libstdc++: Better CTAD for span and mdspan [PR120914].
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.
Reviewed-by: Jonathan Wakely <jwak...@redhat.com>
Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>
Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
Diff:
---
libstdc++-v3/include/std/mdspan | 8 ++-----
libstdc++-v3/include/std/span | 20 ++++++++++++++++-
.../testsuite/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 5a42aead3ebb..5d16de5d9072 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 49ab9109d83e..5629a71b9bd2 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};
+ }
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 e71fdc542305..bca8901685d0 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 9252273bf669..a650fb19bdf0 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 dce6cedf89bd..c66db90222e6 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));
}
