[libcxx-commits] [libcxx] [libc++] Implement P2897R7 aligned_accessor: An mdspan accessor expressing pointer over-alignment (PR #122603)
via libcxx-commits
libcxx-commits at lists.llvm.org
Sat Jan 11 10:27:10 PST 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Damien L-G (dalg24)
<details>
<summary>Changes</summary>
Closes #<!-- -->118372
---
Patch is 40.05 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/122603.diff
21 Files Affected:
- (modified) libcxx/docs/FeatureTestMacroTable.rst (+4)
- (modified) libcxx/docs/Status/Cxx2cPapers.csv (+1-1)
- (modified) libcxx/include/CMakeLists.txt (+2)
- (added) libcxx/include/__mdspan/aligned_accessor.h (+86)
- (modified) libcxx/include/__mdspan/mdspan.h (+5-4)
- (added) libcxx/include/__memory/is_sufficiently_aligned.h (+34)
- (modified) libcxx/include/mdspan (+37-1)
- (modified) libcxx/include/memory (+4)
- (modified) libcxx/include/module.modulemap (+2)
- (modified) libcxx/modules/std/mdspan.inc (+5)
- (modified) libcxx/modules/std/memory.inc (+3)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/access.pass.cpp (+61)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/byte_alignment.verify.cpp (+29)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.conversion.from.default_accessor.pass.cpp (+81)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.conversion.pass.cpp (+104)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.default.pass.cpp (+56)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/element_type.verify.cpp (+32)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/offset.pass.cpp (+61)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/operator.conversion.to.default_accessor.pass.cpp (+80)
- (added) libcxx/test/std/containers/views/mdspan/aligned_accessor/types.pass.cpp (+70)
- (added) libcxx/test/std/utilities/memory/ptr.align/is_sufficiently_aligned.pass.cpp (+105)
``````````diff
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index cfb0e5cfb129ce..ae8cbe61fd7d17 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -406,6 +406,8 @@ Status
----------------------------------------------------------------------------
``__cpp_lib_associative_heterogeneous_insertion`` *unimplemented*
---------------------------------------------------------- -----------------
+ ``__cpp_lib_aligned_accessor`` ``202411L``
+ ---------------------------------------------------------- -----------------
``__cpp_lib_atomic_min_max`` *unimplemented*
---------------------------------------------------------- -----------------
``__cpp_lib_bind_front`` ``202306L``
@@ -450,6 +452,8 @@ Status
---------------------------------------------------------- -----------------
``__cpp_lib_inplace_vector`` *unimplemented*
---------------------------------------------------------- -----------------
+ ``__cpp_lib_is_sufficiently_aligned`` ``202411L``
+ ---------------------------------------------------------- -----------------
``__cpp_lib_is_virtual_base_of`` ``202406L``
---------------------------------------------------------- -----------------
``__cpp_lib_is_within_lifetime`` *unimplemented*
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index aa896e85fcb1fe..ae1f887947fe06 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -81,7 +81,7 @@
"`P3379R0 <https://wg21.link/P3379R0>`__","Constrain ``std::expected`` equality operators","2024-11 (Wrocław)","","",""
"`P0472R2 <https://wg21.link/P0472R2>`__","Put ``std::monostate`` in ``<utility>``","2024-11 (Wrocław)","","",""
"`P2862R1 <https://wg21.link/P2862R1>`__","``text_encoding::name()`` should never return null values","2024-11 (Wrocław)","","",""
-"`P2897R7 <https://wg21.link/P2897R7>`__","``aligned_accessor``: An ``mdspan`` accessor expressing pointer over-alignment","2024-11 (Wrocław)","","",""
+"`P2897R7 <https://wg21.link/P2897R7>`__","``aligned_accessor``: An ``mdspan`` accessor expressing pointer over-alignment","2024-11 (Wrocław)","|Complete|","20",""
"`P3355R1 <https://wg21.link/P3355R1>`__","Fix ``submdspan`` for C++26","2024-11 (Wrocław)","","",""
"`P3222R0 <https://wg21.link/P3222R0>`__","Fix C++26 by adding transposed special cases for P2642 layouts","2024-11 (Wrocław)","","",""
"`P3050R2 <https://wg21.link/P3050R2>`__","Fix C++26 by optimizing ``linalg::conjugated`` for noncomplex value types","2024-11 (Wrocław)","","",""
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index f7721b1047b81e..ad7574b8993d08 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -529,6 +529,7 @@ set(files
__math/traits.h
__math/trigonometric_functions.h
__mbstate_t.h
+ __mdspan/aligned_accessor.h
__mdspan/default_accessor.h
__mdspan/extents.h
__mdspan/layout_left.h
@@ -553,6 +554,7 @@ set(files
__memory/construct_at.h
__memory/destruct_n.h
__memory/inout_ptr.h
+ __memory/is_sufficiently_aligned.h
__memory/noexcept_move_assign_container.h
__memory/out_ptr.h
__memory/pointer_traits.h
diff --git a/libcxx/include/__mdspan/aligned_accessor.h b/libcxx/include/__mdspan/aligned_accessor.h
new file mode 100644
index 00000000000000..441c144c11aebb
--- /dev/null
+++ b/libcxx/include/__mdspan/aligned_accessor.h
@@ -0,0 +1,86 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// Kokkos v. 4.0
+// Copyright (2022) National Technology & Engineering
+// Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MDSPAN_ALIGNED_ACCESSOR_H
+#define _LIBCPP___MDSPAN_ALIGNED_ACCESSOR_H
+
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__memory/assume_aligned.h>
+#include <__type_traits/is_abstract.h>
+#include <__type_traits/is_array.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/remove_const.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <class _ElementType, size_t _ByteAlignment>
+struct aligned_accessor {
+ static_assert(_ByteAlignment != 0 && (_ByteAlignment & (_ByteAlignment - 1)) == 0,
+ "aligned_accessor: byte alignment must be a power of two");
+ static_assert(_ByteAlignment >= alignof(_ElementType), "aligned_accessor: insufficient byte alignment");
+ static_assert(!is_array_v<_ElementType>, "aligned_accessor: template argument may not be an array type");
+ static_assert(!is_abstract_v<_ElementType>, "aligned_accessor: template argument may not be an abstract class");
+
+ using offset_policy = default_accessor<_ElementType>;
+ using element_type = _ElementType;
+ using reference = _ElementType&;
+ using data_handle_type = _ElementType*;
+
+ static constexpr size_t byte_alignment = _ByteAlignment;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr aligned_accessor() noexcept = default;
+
+ template <class _OtherElementType, size_t _OtherByteAlignment>
+ requires(is_convertible_v<_OtherElementType (*)[], element_type (*)[]> && _OtherByteAlignment >= byte_alignment)
+ _LIBCPP_HIDE_FROM_ABI constexpr aligned_accessor(aligned_accessor<_OtherElementType, _OtherByteAlignment>) noexcept {}
+
+ template <class _OtherElementType>
+ requires(is_convertible_v<_OtherElementType (*)[], element_type (*)[]>)
+ _LIBCPP_HIDE_FROM_ABI explicit constexpr aligned_accessor(default_accessor<_OtherElementType>) noexcept {}
+
+ template <class _OtherElementType>
+ requires(is_convertible_v<element_type (*)[], _OtherElementType (*)[]>)
+ _LIBCPP_HIDE_FROM_ABI constexpr operator default_accessor<_OtherElementType>() const noexcept {
+ return {};
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr reference access(data_handle_type __p, size_t __i) const noexcept {
+ return std::assume_aligned<byte_alignment>(__p)[__i];
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr typename offset_policy::data_handle_type
+ offset(data_handle_type __p, size_t __i) const noexcept {
+ return std::assume_aligned<byte_alignment>(__p) + __i;
+ }
+};
+
+#endif // _LIBCPP_STD_VER >= 26
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___MDSPAN_ALIGNED_ACCESSOR_H
diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index 3f9b35b185b167..ef8db9f1f9789a 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -20,6 +20,7 @@
#include <__assert>
#include <__config>
#include <__fwd/mdspan.h>
+#include <__mdspan/aligned_accessor.h>
#include <__mdspan/default_accessor.h>
#include <__mdspan/extents.h>
#include <__type_traits/extent.h>
@@ -266,13 +267,13 @@ 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>...>>;
+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)>>;
+explicit mdspan(_ElementType*, _OtherIndexTypes...)
+ -> mdspan<_ElementType, dextents<size_t, sizeof...(_OtherIndexTypes)>>;
# endif
template <class _Pointer>
diff --git a/libcxx/include/__memory/is_sufficiently_aligned.h b/libcxx/include/__memory/is_sufficiently_aligned.h
new file mode 100644
index 00000000000000..4280920cabb4bc
--- /dev/null
+++ b/libcxx/include/__memory/is_sufficiently_aligned.h
@@ -0,0 +1,34 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H
+#define _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H
+
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <size_t _Alignment, class _Tp>
+_LIBCPP_HIDE_FROM_ABI bool is_sufficiently_aligned(_Tp* __ptr) {
+ return reinterpret_cast<uintptr_t>(__ptr) % _Alignment == 0;
+}
+
+#endif // _LIBCPP_STD_VER >= 26
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H
diff --git a/libcxx/include/mdspan b/libcxx/include/mdspan
index a67ed1834e3bde..61f1b210dd03af 100644
--- a/libcxx/include/mdspan
+++ b/libcxx/include/mdspan
@@ -33,10 +33,14 @@ namespace std {
template<class ElementType>
class default_accessor;
+ // [mdspan.accessor.aligned], class template aligned_accessor
+ template<class ElementType, size_t ByteAlignment>
+ class aligned_accessor; // since C++26
+
// [mdspan.mdspan], class template mdspan
template<class ElementType, class Extents, class LayoutPolicy = layout_right,
class AccessorPolicy = default_accessor<ElementType>>
- class mdspan; // not implemented yet
+ class mdspan;
}
// extents synopsis
@@ -269,6 +273,38 @@ namespace std {
};
}
+// aligned_accessor synopsis
+
+namespace std {
+ template<class ElementType, size_t ByteAlignment>
+ struct aligned_accessor {
+ using offset_policy = default_accessor<ElementType>;
+ using element_type = ElementType;
+ using reference = ElementType&;
+ using data_handle_type = ElementType*;
+
+ static constexpr size_t byte_alignment = ByteAlignment;
+
+ constexpr aligned_accessor() noexcept = default;
+
+ template<class OtherElementType, size_t OtherByteAlignment>
+ constexpr aligned_accessor(
+ aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;
+
+ template<class OtherElementType>
+ explicit constexpr aligned_accessor(
+ default_accessor<OtherElementType>) noexcept;
+
+ template<class OtherElementType>
+ constexpr operator default_accessor<OtherElementType>() const noexcept;
+
+ constexpr reference access(data_handle_type p, size_t i) const noexcept;
+
+ constexpr typename offset_policy::data_handle_type
+ offset(data_handle_type p, size_t i) const noexcept;
+ };
+}
+
// mdspan synopsis
namespace std {
diff --git a/libcxx/include/memory b/libcxx/include/memory
index fc62606ea0fd33..a6400bd29f4437 100644
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -912,6 +912,9 @@ void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
template<size_t N, class T>
[[nodiscard]] constexpr T* assume_aligned(T* ptr); // since C++20
+template<size_t Alignment, class T>
+ bool is_sufficiently_aligned(T* ptr); // since C++26
+
// [out.ptr.t], class template out_ptr_t
template<class Smart, class Pointer, class... Args>
class out_ptr_t; // since c++23
@@ -945,6 +948,7 @@ template<class Pointer = void, class Smart, class... Args>
# include <__memory/allocator_traits.h>
# include <__memory/auto_ptr.h>
# include <__memory/inout_ptr.h>
+# include <__memory/is_sufficiently_aligned.h>
# include <__memory/out_ptr.h>
# include <__memory/pointer_traits.h>
# include <__memory/raw_storage_iterator.h>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 07ab5649ae45cb..c4beb2dfa4f383 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -1504,6 +1504,7 @@ module std [system] {
}
module mdspan {
+ module aligned_accessor { header "__mdspan/aligned_accessor.h" }
module default_accessor { header "__mdspan/default_accessor.h" }
module extents { header "__mdspan/extents.h" }
module fwd { header "__fwd/mdspan.h" }
@@ -1539,6 +1540,7 @@ module std [system] {
module destruct_n { header "__memory/destruct_n.h" }
module fwd { header "__fwd/memory.h" }
module inout_ptr { header "__memory/inout_ptr.h" }
+ module is_sufficiently_aligned { header "__memory/is_sufficiently_aligned.h" }
module noexcept_move_assign_container { header "__memory/noexcept_move_assign_container.h" }
module out_ptr { header "__memory/out_ptr.h" }
module pointer_traits { header "__memory/pointer_traits.h" }
diff --git a/libcxx/modules/std/mdspan.inc b/libcxx/modules/std/mdspan.inc
index 5e65993383cdc0..c8cc78badb7d59 100644
--- a/libcxx/modules/std/mdspan.inc
+++ b/libcxx/modules/std/mdspan.inc
@@ -28,6 +28,11 @@ export namespace std {
// [mdspan.accessor.default], class template default_accessor
using std::default_accessor;
+# if _LIBCPP_STD_VER >= 26
+ // [mdspan.accessor.aligned], class template aligned_accessor
+ using std::aligned_accessor;
+# endif // _LIBCPP_STD_VER >= 26
+
// [mdspan.mdspan], class template mdspan
using std::mdspan;
#endif // _LIBCPP_STD_VER >= 23
diff --git a/libcxx/modules/std/memory.inc b/libcxx/modules/std/memory.inc
index 82056e426d06c9..c25e9e3443e9cf 100644
--- a/libcxx/modules/std/memory.inc
+++ b/libcxx/modules/std/memory.inc
@@ -17,6 +17,9 @@ export namespace std {
// [ptr.align], pointer alignment
using std::align;
using std::assume_aligned;
+#if _LIBCPP_STD_VER >= 26
+ using std::is_sufficiently_aligned;
+#endif
// [obj.lifetime], explicit lifetime management
// using std::start_lifetime_as;
diff --git a/libcxx/test/std/containers/views/mdspan/aligned_accessor/access.pass.cpp b/libcxx/test/std/containers/views/mdspan/aligned_accessor/access.pass.cpp
new file mode 100644
index 00000000000000..145dc79eda73d4
--- /dev/null
+++ b/libcxx/test/std/containers/views/mdspan/aligned_accessor/access.pass.cpp
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <mdspan>
+
+// constexpr reference access(data_handle_type p, size_t i) const noexcept;
+//
+// Effects: Equivalent to: return assume_aligned<byte_alignment>(p)[i];
+
+#include <mdspan>
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+
+template <class T, size_t N>
+constexpr void test_access() {
+ ElementPool<std::remove_const_t<T>, 10 + N> data;
+ T* ptr = data.get_ptr();
+ std::aligned_accessor<T, N> acc;
+ for (size_t i = 0; i < 10 + N; i++) {
+ if (reinterpret_cast<std::uintptr_t>(ptr + i) % N == 0) {
+ std::same_as<typename std::aligned_accessor<T, N>::reference> decltype(auto) x = acc.access(ptr, i);
+ ASSERT_NOEXCEPT(acc.access(ptr, i));
+ assert(&x == ptr + i);
+ }
+ }
+}
+
+template <class T>
+constexpr void test_it() {
+ constexpr size_t N = alignof(T);
+ test_access<T, N>();
+ test_access<T, 2 * N>();
+ test_access<T, 4 * N>();
+ test_access<T, 8 * N>();
+ test_access<T, 16 * N>();
+}
+
+constexpr bool test() {
+ test_it<int>();
+ test_it<const int>();
+ test_it<MinimalElementType>();
+ test_it<const MinimalElementType>();
+ return true;
+}
+
+int main(int, char**) {
+ test();
+ //static_assert(test());
+ return 0;
+}
diff --git a/libcxx/test/std/containers/views/mdspan/aligned_accessor/byte_alignment.verify.cpp b/libcxx/test/std/containers/views/mdspan/aligned_accessor/byte_alignment.verify.cpp
new file mode 100644
index 00000000000000..fbc8386a72dede
--- /dev/null
+++ b/libcxx/test/std/containers/views/mdspan/aligned_accessor/byte_alignment.verify.cpp
@@ -0,0 +1,29 @@
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <mdspan>
+
+// template<class ElementType, size_t ByteAlignment>
+// class aligned_accessor;
+
+// ByteAlignement is required to be a power of two and greater or equal to alignof(ElementType).
+
+#include <mdspan>
+
+void not_power_of_two() {
+ // expected-error-re@*:* {{static assertion failed {{.*}}aligned_accessor: byte alignment must be a power of two}}
+ [[maybe_unused]] std::aligned_accessor<int, 12> acc;
+}
+
+struct alignas(8) S {};
+
+void insufficiently_aligned() {
+ // expected-error-re@*:* {{static assertion failed {{.*}}aligned_accessor: insufficient byte alignment}}
+ [[maybe_unused]] std::aligned_accessor<S, 4> acc;
+}
diff --git a/libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.conversion.from.default_accessor.pass.cpp b/libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.conversion.from.default_accessor.pass.cpp
new file mode 100644
index 00000000000000..339c12bb7c6c54
--- /dev/null
+++ b/libcxx/test/std/containers/views/mdspan/aligned_accessor/ctor.conversion.from.default_accessor.pass.cpp
@@ -0,0 +1,81 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <mdspan>
+//
+// template<class OtherElementType>
+// explicit constexpr aligned_accessor(
+// default_accessor<OtherElementType>) noexcept {}
+//
+// Constraints: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+
+struct Base {};
+struct Derived : public Base {};
+
+template <class FromT, class ToT, size_t ToN>
+constexpr void test_conversion() {
+ std::default_accessor<FromT> from;
+ ASSERT_NOEXCEPT(std::aligned_accessor<ToT, ToN>(from));
+ [[maybe_unused]] std::aligned_accessor<ToT, ToN> to(from);
+ // check that the constructor is explicit
+ static_assert(std::is_nothrow_constructible_v<std::aligned_accessor<ToT, ToN>, std::default_accessor<ToT>>);
+ static_assert(!std::is_convertible_v<std::default_accessor<ToT>, std::aligned_accessor<ToT, ToN>>);
+}
+
+template <class From, class To>
+constexpr void test_it() {
+ constexpr size_t N = alignof(To);
+ test_conversion<From, To, N>();
+ test_conversion<From, To, 2 * N>();
+ test_conversion<From, To, 4 * N>();
+ test_conversion<From, To, 8 * N>();
+ test_conversion<From, To, 16 * N>();
+ test_conversion<From, To, 32 * N>(...
[truncated]
``````````
</details>
https://github.com/llvm/llvm-project/pull/122603
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