[PATCH v1] libstdc++: Better CTAD for span and mdspan [P3029].

[Luc Grosheintz]

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.

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/p3029.cc: New test.
	* testsuite/23_containers/mdspan/p3029.cc: New test.

Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
---
 libstdc++-v3/include/std/mdspan               |  4 +-
 libstdc++-v3/include/std/span                 | 20 ++++++-
 .../testsuite/23_containers/mdspan/p3029.cc   | 58 +++++++++++++++++++
 .../testsuite/23_containers/span/p3029.cc     | 11 ++++
 4 files changed, 90 insertions(+), 3 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/p3029.cc
 create mode 100644 libstdc++-v3/testsuite/23_containers/span/p3029.cc

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index f64804e2a42..51abf9624e0 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -419,7 +419,7 @@ _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
   {
@@ -1315,7 +1315,7 @@ _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};
+  }
 
   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/p3029.cc b/libstdc++-v3/testsuite/23_containers/mdspan/p3029.cc
new file mode 100644
index 00000000000..7c38dc00bf4
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/p3029.cc
@@ -0,0 +1,58 @@
+// { dg-do run { target c++23 } }
+
+#include <mdspan>
+
+#include <testsuite_hooks.h>
+
+constexpr size_t dyn = std::dynamic_extent;
+
+constexpr bool
+test_extents()
+{
+  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_mdspan()
+{
+  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;
+}
+
+int
+main()
+{
+  test_extents();
+  static_assert(test_extents());
+
+  test_mdspan();
+  static_assert(test_mdspan());
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/p3029.cc b/libstdc++-v3/testsuite/23_containers/span/p3029.cc
new file mode 100644
index 00000000000..e894255546f
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/p3029.cc
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++20 } }
+
+#include <span>
+
+constexpr void
+test_ctad()
+{
+  std::array<double, 5> a{};
+  std::span s(a.data(), std::integral_constant<size_t, 5>{});
+  static_assert(std::same_as<decltype(s), std::span<double, 5>>);
+}
-- 
2.49.0