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

Xiaoyang Liu via libcxx-commits libcxx-commits at lists.llvm.org
Sat Apr 6 03:22:46 PDT 2024 

https://github.com/xiaoyang-sde updated https://github.com/llvm/llvm-project/pull/87873

>From 386a5dd1255ae87798c80265466f6bfbcb2edcde Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 01:58:44 -0700
Subject: [PATCH 1/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/docs/ReleaseNotes/19.rst               |  1 +
 libcxx/docs/Status/Cxx2cPapers.csv            |  2 +-
 libcxx/include/__fwd/span.h                   | 23 +++++++++++++++++++
 libcxx/include/__mdspan/mdspan.h              |  8 +++++++
 libcxx/include/mdspan                         |  6 ++++-
 libcxx/include/span                           | 23 ++++++++++++++++++-
 .../views/mdspan/mdspan/deduction.pass.cpp    | 18 ++++++++++++++-
 .../views.span/span.cons/deduct.pass.cpp      | 11 +++++++++
 8 files changed, 88 insertions(+), 4 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index abf1570737df9e..4a1fca4ce6db9c 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -46,6 +46,7 @@ Implemented Papers
 - P2872R3 - Remove ``wstring_convert`` From C++26
 - 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 af270150fda0c0..f707d996a26ea8 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/__fwd/span.h b/libcxx/include/__fwd/span.h
index 8dafa742c19df5..2a438254bc4468 100644
--- a/libcxx/include/__fwd/span.h
+++ b/libcxx/include/__fwd/span.h
@@ -10,7 +10,13 @@
 #ifndef _LIBCPP___FWD_SPAN_H
 #define _LIBCPP___FWD_SPAN_H
 
+#include <__concepts/convertible_to.h>
+#include <__concepts/equality_comparable.h>
 #include <__config>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_integral.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/remove_const.h>
 #include <cstddef>
 #include <limits>
 
@@ -31,6 +37,23 @@ class span;
 
 #endif
 
+#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>
+constexpr size_t __maybe_static_ext = dynamic_extent;
+
+template <__integral_constant_like _Tp>
+constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
+
+#endif // _LIBCPP_STD_VER >= 26
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index d9a0108d6d3507..ed029ae0a0ea7a 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -17,6 +17,7 @@
 #ifndef _LIBCPP___MDSPAN_MDSPAN_H
 #define _LIBCPP___MDSPAN_MDSPAN_H
 
+#include "__fwd/span.h"
 #include <__assert>
 #include <__config>
 #include <__fwd/mdspan.h>
@@ -266,10 +267,17 @@ class mdspan {
   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, dextents<size_t, sizeof...(_OtherIndexTypes)>>;
+#  else
+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>...>>;
+#  endif
 
 template <class _Pointer>
   requires(is_pointer_v<remove_reference_t<_Pointer>>)
diff --git a/libcxx/include/mdspan b/libcxx/include/mdspan
index c13d9eef001ac9..76f34b67e41004 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, dextents<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 c0fe25ddb4beb9..89cf334f58058b 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>;
@@ -563,8 +579,13 @@ _LIBCPP_HIDE_FROM_ABI auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept {
   return __s.__as_writable_bytes();
 }
 
+#if _LIBCPP_STD_VER < 26
 template <contiguous_iterator _It, class _EndOrSize>
 span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
+#else
+template <contiguous_iterator _It, class _EndOrSize>
+span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>, __maybe_static_ext<_EndOrSize>>;
+#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 557829dcbad9be..8d3a86910e33d7 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, dextents<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 398862dccdb6a1..92bb7c9caea836 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() {

>From f1dc5790288454a14f9f95161d5deab98b89571c Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 02:01:00 -0700
Subject: [PATCH 2/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/include/span | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/span b/libcxx/include/span
index 89cf334f58058b..ce05009bfe3ed3 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -19,7 +19,7 @@ namespace std {
 inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
 
 template<class T>
-  concept integral-constant-like =        // exposition only, since C++26
+  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)> &&

>From 730a76d3693abb5394315fd3f9210bd2e719b691 Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 02:01:50 -0700
Subject: [PATCH 3/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/include/span | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/span b/libcxx/include/span
index ce05009bfe3ed3..7c430036478d55 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -124,7 +124,7 @@ private:
 };
 
 template<class It, class EndOrSize>
-    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>; // until C++26
+    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
 

>From 6ac8a6cbde03499acdbbef8855812d3f1f07c332 Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 02:11:05 -0700
Subject: [PATCH 4/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/include/__mdspan/mdspan.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index ed029ae0a0ea7a..7d57bf199f4ac3 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -275,8 +275,8 @@ explicit mdspan(_ElementType*, _OtherIndexTypes...)
 #  else
 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>...>>;
+explicit mdspan(_ElementType*,
+                _OtherIndexTypes...) -> mdspan<_ElementType, extents<size_t, __maybe_static_ext<_OtherIndexTypes>...>>;
 #  endif
 
 template <class _Pointer>

>From aa4049433d41553d57c229690f5595a9b9022646 Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 03:19:51 -0700
Subject: [PATCH 5/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/include/__fwd/span.h | 25 +------------------------
 libcxx/include/span         | 29 ++++++++++++++++++++++++++---
 2 files changed, 27 insertions(+), 27 deletions(-)

diff --git a/libcxx/include/__fwd/span.h b/libcxx/include/__fwd/span.h
index 2a438254bc4468..fbf942dda177b2 100644
--- a/libcxx/include/__fwd/span.h
+++ b/libcxx/include/__fwd/span.h
@@ -1,4 +1,4 @@
-// -*- C++ -*-
+// -*- C++ -*// -*- C++ -*-
 //===---------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
@@ -10,13 +10,7 @@
 #ifndef _LIBCPP___FWD_SPAN_H
 #define _LIBCPP___FWD_SPAN_H
 
-#include <__concepts/convertible_to.h>
-#include <__concepts/equality_comparable.h>
 #include <__config>
-#include <__type_traits/integral_constant.h>
-#include <__type_traits/is_integral.h>
-#include <__type_traits/is_same.h>
-#include <__type_traits/remove_const.h>
 #include <cstddef>
 #include <limits>
 
@@ -37,23 +31,6 @@ class span;
 
 #endif
 
-#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>
-constexpr size_t __maybe_static_ext = dynamic_extent;
-
-template <__integral_constant_like _Tp>
-constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
-
-#endif // _LIBCPP_STD_VER >= 26
-
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/libcxx/include/span b/libcxx/include/span
index 7c430036478d55..55e3ea24089e64 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -145,6 +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>
@@ -159,9 +161,13 @@ 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>
@@ -190,6 +196,23 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#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>
+constexpr size_t __maybe_static_ext = dynamic_extent;
+
+template <__integral_constant_like _Tp>
+constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
+
+#endif // _LIBCPP_STD_VER >= 26
+
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp>
@@ -579,13 +602,13 @@ _LIBCPP_HIDE_FROM_ABI auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept {
   return __s.__as_writable_bytes();
 }
 
-#if _LIBCPP_STD_VER < 26
+#  if _LIBCPP_STD_VER < 26
 template <contiguous_iterator _It, class _EndOrSize>
 span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
-#else
+#  else
 template <contiguous_iterator _It, class _EndOrSize>
 span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>, __maybe_static_ext<_EndOrSize>>;
-#endif
+#  endif
 
 template <class _Tp, size_t _Sz>
 span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;

>From a7ff0a05fa809c5f62c5523cc9671735c32a2f73 Mon Sep 17 00:00:00 2001
From: Xiaoyang Liu <siujoeng.lau at gmail.com>
Date: Sat, 6 Apr 2024 03:22:32 -0700
Subject: [PATCH 6/6] [libc++] P3029R1: Better mdspan's CTAD

---
 libcxx/include/__fwd/span.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/__fwd/span.h b/libcxx/include/__fwd/span.h
index fbf942dda177b2..8dafa742c19df5 100644
--- a/libcxx/include/__fwd/span.h
+++ b/libcxx/include/__fwd/span.h
@@ -1,4 +1,4 @@
-// -*- C++ -*// -*- C++ -*-
+// -*- C++ -*-
 //===---------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

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