[libc++] P3029R1: Better `mdspan`'s CTAD (#87873)

## Abstract

This pull request implements [P3029R1](https://wg21.link/P3029R1). The
paper discusses the current behavior of `mdspan`'s most common
pointer-indices CTAD, where the `Extents` template parameter is deduced
as `dextents` (dynamic extents), even when passing compile-time constant
values. The author believes this behavior is suboptimal, as it doesn't
take advantage of the compile-time information. The proposed change
suggests deducing static extents if `integral_constant`-like constants
are passed, resulting in more intuitive syntax and less error-prone
code.

## Reference

- [P3029R1](https://wg21.link/P3029R1)
- [Draft C++ Standard: [span.syn]](https://eel.is/c++draft/span.syn)
- [Draft C++ Standard: [mdspan.syn]](https://eel.is/c++draft/mdspan.syn)

7 files changed, 84 insertions, 5 deletions

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 81c05b9..7bc0148 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -47,6 +47,7 @@ Implemented Papers
 - P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
 - P2302R4 - ``std::ranges::contains``
 - P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
+- P3029R1 - Better ``mdspan``'s CTAD
 
 Improvements and New Features
 -----------------------------
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index fa11da6..8516070 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -61,5 +61,5 @@
 "`P1068R11 <https://wg21.link/P1068R11>`__","LWG","Vector API for random number generation","Tokyo March 2024","","",""
 "`P2944R3 <https://wg21.link/P2944R3>`__","LWG","Comparisons for ``reference_wrapper``","Tokyo March 2024","","",""
 "`P2642R6 <https://wg21.link/P2642R6>`__","LWG","Padded ``mdspan`` layouts","Tokyo March 2024","","",""
-"`P3029R1 <https://wg21.link/P3029R1>`__","LWG","Better ``mdspan``'s CTAD","Tokyo March 2024","","",""
+"`P3029R1 <https://wg21.link/P3029R1>`__","LWG","Better ``mdspan``'s CTAD","Tokyo March 2024","|Complete|","19.0",""
 "","","","","","",""
diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index d9a0108..09665af 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -266,10 +266,17 @@ private:
   friend class mdspan;
 };
 
+#  if _LIBCPP_STD_VER >= 26
+template <class _ElementType, class... _OtherIndexTypes>
+  requires((is_convertible_v<_OtherIndexTypes, size_t> && ...) && (sizeof...(_OtherIndexTypes) > 0))
+explicit mdspan(_ElementType*,
+                _OtherIndexTypes...) -> mdspan<_ElementType, extents<size_t, __maybe_static_ext<_OtherIndexTypes>...>>;
+#  else
 template <class _ElementType, class... _OtherIndexTypes>
   requires((is_convertible_v<_OtherIndexTypes, size_t> && ...) && (sizeof...(_OtherIndexTypes) > 0))
 explicit mdspan(_ElementType*, _OtherIndexTypes...)
     -> mdspan<_ElementType, dextents<size_t, sizeof...(_OtherIndexTypes)>>;
+#  endif
 
 template <class _Pointer>
   requires(is_pointer_v<remove_reference_t<_Pointer>>)
diff --git a/libcxx/include/mdspan b/libcxx/include/mdspan
index c13d9ee..8d443f4 100644
--- a/libcxx/include/mdspan
+++ b/libcxx/include/mdspan
@@ -370,7 +370,11 @@ namespace std {
   template<class ElementType, class... Integrals>
     requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
     explicit mdspan(ElementType*, Integrals...)
-      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
+      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;            // until C++26
+  template<class ElementType, class... Integrals>
+    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
+    explicit mdspan(ElementType*, Integrals...)
+      -> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>;  // since C++26
 
   template<class ElementType, class OtherIndexType, size_t N>
     mdspan(ElementType*, span<OtherIndexType, N>)
diff --git a/libcxx/include/span b/libcxx/include/span
index c0fe25d..0307edc 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -18,6 +18,20 @@ namespace std {
 // constants
 inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
 
+template<class T>
+  concept integral-constant-like =                           // exposition only, since C++26
+    is_integral_v<decltype(T::value)> &&
+    !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
+    convertible_to<T, decltype(T::value)> &&
+    equality_comparable_with<T, decltype(T::value)> &&
+    bool_constant<T() == T::value>::value &&
+    bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
+
+template<class T>
+  constexpr size_t maybe-static-ext = dynamic_extent;        // exposition only, since C++26
+template<integral-constant-like T>
+  constexpr size_t maybe-static-ext<T> = {T::value};
+
 // [views.span], class template span
 template <class ElementType, size_t Extent = dynamic_extent>
     class span;
@@ -110,7 +124,9 @@ private:
 };
 
 template<class It, class EndOrSize>
-    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
+    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;                             // until C++26
+template<class It, class EndOrSize>
+    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>, maybe-static-ext<EndOrSize>>; // since C++26
 
 template<class T, size_t N>
     span(T (&)[N]) -> span<T, N>;
@@ -129,6 +145,8 @@ template<class R>
 */
 
 #include <__assert>
+#include <__concepts/convertible_to.h>
+#include <__concepts/equality_comparable.h>
 #include <__config>
 #include <__fwd/array.h>
 #include <__fwd/span.h>
@@ -143,15 +161,19 @@ template<class R>
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/enable_view.h>
 #include <__ranges/size.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_array.h>
 #include <__type_traits/is_const.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_integral.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/remove_const.h>
 #include <__type_traits/remove_cv.h>
 #include <__type_traits/remove_cvref.h>
 #include <__type_traits/remove_reference.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/forward.h>
-#include <cstddef>      // for byte
+#include <cstddef> // for byte
 #include <initializer_list>
 #include <stdexcept>
 #include <version>
@@ -563,8 +585,26 @@ _LIBCPP_HIDE_FROM_ABI auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept {
   return __s.__as_writable_bytes();
 }
 
+#  if _LIBCPP_STD_VER >= 26
+template <class _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 <class _Tp>
+inline constexpr size_t __maybe_static_ext = dynamic_extent;
+
+template <__integral_constant_like _Tp>
+inline constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
+
+template <contiguous_iterator _It, class _EndOrSize>
+span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>, __maybe_static_ext<_EndOrSize>>;
+#  else
 template <contiguous_iterator _It, class _EndOrSize>
 span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
+#  endif
 
 template <class _Tp, size_t _Sz>
 span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;
diff --git a/libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.cpp b/libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.cpp
index 557829d..876a3e8 100644
--- a/libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.cpp
@@ -22,7 +22,11 @@
 //  template<class ElementType, class... Integrals>
 //    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
 //    explicit mdspan(ElementType*, Integrals...)
-//      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
+//      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;            // until C++26
+//  template<class ElementType, class... Integrals>
+//    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
+//    explicit mdspan(ElementType*, Integrals...)
+//      -> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>;  // since C++26
 //
 //  template<class ElementType, class OtherIndexType, size_t N>
 //    mdspan(ElementType*, span<OtherIndexType, N>)
@@ -102,6 +106,18 @@ constexpr bool test_no_layout_deduction_guides(const H& handle, const A&) {
   // deduction from pointer and integral like
   ASSERT_SAME_TYPE(decltype(std::mdspan(handle, 5, SizeTIntType(6))), std::mdspan<T, std::dextents<size_t, 2>>);
 
+#if _LIBCPP_STD_VER >= 26
+  // P3029R1: deduction from `integral_constant`
+  ASSERT_SAME_TYPE(
+      decltype(std::mdspan(handle, std::integral_constant<size_t, 5>{})), std::mdspan<T, std::extents<size_t, 5>>);
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle, std::integral_constant<size_t, 5>{}, std::dynamic_extent)),
+                   std::mdspan<T, std::extents<size_t, 5, std::dynamic_extent>>);
+  ASSERT_SAME_TYPE(
+      decltype(std::mdspan(
+          handle, std::integral_constant<size_t, 5>{}, std::dynamic_extent, std::integral_constant<size_t, 7>{})),
+      std::mdspan<T, std::extents<size_t, 5, std::dynamic_extent, 7>>);
+#endif
+
   std::array<char, 3> exts;
   // deduction from pointer and array
   ASSERT_SAME_TYPE(decltype(std::mdspan(handle, exts)), std::mdspan<T, std::dextents<size_t, 3>>);
diff --git a/libcxx/test/std/containers/views/views.span/span.cons/deduct.pass.cpp b/libcxx/test/std/containers/views/views.span/span.cons/deduct.pass.cpp
index 398862d..92bb7c9 100644
--- a/libcxx/test/std/containers/views/views.span/span.cons/deduct.pass.cpp
+++ b/libcxx/test/std/containers/views/views.span/span.cons/deduct.pass.cpp
@@ -31,6 +31,7 @@
 #include <iterator>
 #include <memory>
 #include <string>
+#include <type_traits>
 
 #include "test_macros.h"
 
@@ -48,6 +49,16 @@ void test_iterator_sentinel() {
   assert(s.size() == std::size(arr));
   assert(s.data() == std::data(arr));
   }
+
+#if _LIBCPP_STD_VER >= 26
+  // P3029R1: deduction from `integral_constant`
+  {
+    std::span s{std::begin(arr), std::integral_constant<size_t, 3>{}};
+    ASSERT_SAME_TYPE(decltype(s), std::span<int, 3>);
+    assert(s.size() == std::size(arr));
+    assert(s.data() == std::data(arr));
+  }
+#endif
 }
 
 void test_c_array() {