[libcxx-commits] [libcxx] 5588749 - [libc++] Implement LWG4290: Missing Mandates clauses on is_sufficiently_aligned (#229765)

[via libcxx-commits]

Author: Karolishp
Date: 2026-10-10T21:36:00+08:00
New Revision: 5588749b29b2cd3959a052a56454678495761ecb

URL: https://github.com/llvm/llvm-project/commit/5588749b29b2cd3959a052a56454678495761ecb
DIFF: https://github.com/llvm/llvm-project/commit/5588749b29b2cd3959a052a56454678495761ecb.diff

LOG: [libc++] Implement LWG4290: Missing Mandates clauses on is_sufficiently_aligned (#229765)

LWG4290 adds a Mandates clause that requires `Alignment` to be a power
of two for `std::is_sufficiently_aligned`. This patch adds the power of
two check as a `static_assert` in the internal helper of
`std::is_sufficiently_aligned`. The change mirrors
`std::assume_aligned`. I added a new libc++ verify test.

Fixes #189823.

## AI Disclosure

AI was used to review the changes before putting this PR up.

Added: 
    libcxx/test/libcxx/utilities/memory/ptr.align/is_sufficiently_aligned.power2.verify.cpp

Modified: 
    libcxx/docs/Status/Cxx26Issues.csv
    libcxx/include/__memory/is_sufficiently_aligned.h

Removed: 
    


################################################################################
diff  --git a/libcxx/docs/Status/Cxx26Issues.csv b/libcxx/docs/Status/Cxx26Issues.csv
index 7cb257a49c30f..9f2a8edbde8c9 100644
--- a/libcxx/docs/Status/Cxx26Issues.csv
+++ b/libcxx/docs/Status/Cxx26Issues.csv
@@ -278,7 +278,7 @@
 "`LWG4151 <https://wg21.link/LWG4151>`__","Precondition of ``inplace_vector::swap``","2026-03 (Croydon)","","","`#189820 <https://github.com/llvm/llvm-project/issues/189820>`__",""
 "`LWG4223 <https://wg21.link/LWG4223>`__","Deduction guides for maps are mishandling tuples and references","2026-03 (Croydon)","","","`#189821 <https://github.com/llvm/llvm-project/issues/189821>`__",""
 "`LWG4259 <https://wg21.link/LWG4259>`__","P1148R0 changed the return values of searching functions of ``std::basic_string`` on some platforms","2026-03 (Croydon)","","","`#189822 <https://github.com/llvm/llvm-project/issues/189822>`__",""
-"`LWG4290 <https://wg21.link/LWG4290>`__","Missing *Mandates* clauses on ``is_sufficiently_aligned``","2026-03 (Croydon)","","","`#189823 <https://github.com/llvm/llvm-project/issues/189823>`__",""
+"`LWG4290 <https://wg21.link/LWG4290>`__","Missing *Mandates* clauses on ``is_sufficiently_aligned``","2026-03 (Croydon)","|Complete|","24","`#189823 <https://github.com/llvm/llvm-project/issues/189823>`__",""
 "`LWG4314 <https://wg21.link/LWG4314>`__","Missing move in ``mdspan`` layout ``mapping::operator()``","2026-03 (Croydon)","","","`#189824 <https://github.com/llvm/llvm-project/issues/189824>`__",""
 "`LWG4324 <https://wg21.link/LWG4324>`__","``unique_ptr<void>::operator*`` is not SFINAE-friendly","2026-03 (Croydon)","|Complete|","23","`#189825 <https://github.com/llvm/llvm-project/issues/189825>`__",""
 "`LWG4325 <https://wg21.link/LWG4325>`__","``std::indirect``'s ``operator==`` still does not support incomplete types","2026-03 (Croydon)","","","`#189826 <https://github.com/llvm/llvm-project/issues/189826>`__",""

diff  --git a/libcxx/include/__memory/is_sufficiently_aligned.h b/libcxx/include/__memory/is_sufficiently_aligned.h
index b8725880028db..587710d7e3526 100644
--- a/libcxx/include/__memory/is_sufficiently_aligned.h
+++ b/libcxx/include/__memory/is_sufficiently_aligned.h
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H
 #define _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H
 
+#include <__bit/has_single_bit.h>
 #include <__config>
 #include <__cstddef/size_t.h>
 #include <cstdint>
@@ -18,22 +19,28 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <size_t _Alignment, class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool __is_sufficiently_aligned(_Tp* __ptr) {
+  static_assert(std::has_single_bit(_Alignment), "alignment must be a power of two.");
+
   return reinterpret_cast<uintptr_t>(__ptr) % _Alignment == 0;
 }
 
-#if _LIBCPP_STD_VER >= 26
+#  if _LIBCPP_STD_VER >= 26
 
 template <size_t _Alignment, class _Tp>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool is_sufficiently_aligned(_Tp* __ptr) {
   return std::__is_sufficiently_aligned<_Alignment>(__ptr);
 }
 
-#endif // _LIBCPP_STD_VER >= 26
+#  endif // _LIBCPP_STD_VER >= 26
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___MEMORY_IS_SUFFICIENTLY_ALIGNED_H

diff  --git a/libcxx/test/libcxx/utilities/memory/ptr.align/is_sufficiently_aligned.power2.verify.cpp b/libcxx/test/libcxx/utilities/memory/ptr.align/is_sufficiently_aligned.power2.verify.cpp
new file mode 100644
index 0000000000000..cde7aa38deb74
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/memory/ptr.align/is_sufficiently_aligned.power2.verify.cpp
@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// #include <memory>
+
+// template<size_t Alignment, class T>
+//   bool is_sufficiently_aligned(T* ptr);
+
+#include <memory>
+
+void f() {
+  int* p = nullptr;
+  (void)std::is_sufficiently_aligned<0>(p);  // expected-error@*:* {{alignment must be a power of two.}}
+  (void)std::is_sufficiently_aligned<3>(p);  // expected-error@*:* {{alignment must be a power of two.}}
+  (void)std::is_sufficiently_aligned<5>(p);  // expected-error@*:* {{alignment must be a power of two.}}
+  (void)std::is_sufficiently_aligned<33>(p); // expected-error@*:* {{alignment must be a power of two.}}
+}