[libcxx-commits] [libcxx] [libc++][C++03] Cherry-pick #106832 (PR #205547)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/205547

>From 4ff9bec3a5f410395e8388472a924a9cb0559239 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Wed, 24 Jun 2026 14:44:40 +0200
Subject: [PATCH] [libc++][C++03] Cherry-pick #106832

---
 libcxx/include/__cxx03/__config               | 14 +-------------
 .../__cxx03/__type_traits/datasizeof.h        | 19 +++----------------
 .../type_traits/datasizeof.compile.pass.cpp   |  4 +---
 3 files changed, 5 insertions(+), 32 deletions(-)

diff --git a/libcxx/include/__cxx03/__config b/libcxx/include/__cxx03/__config
index 27c26084da4b1..c7b2d2b1a7c6d 100644
--- a/libcxx/include/__cxx03/__config
+++ b/libcxx/include/__cxx03/__config
@@ -842,21 +842,9 @@ typedef __char32_t char32_t;
 // (If/when MSVC breaks its C++ ABI, it will be changed to work as intended.)
 // However, MSVC implements [[msvc::no_unique_address]] which does what
 // [[no_unique_address]] is supposed to do, in general.
-
-// Clang-cl does not yet (14.0) implement either [[no_unique_address]] or
-// [[msvc::no_unique_address]] though. If/when it does implement
-// [[msvc::no_unique_address]], this should be preferred though.
 #    define _LIBCPP_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
-#  elif __has_cpp_attribute(no_unique_address)
-#    define _LIBCPP_NO_UNIQUE_ADDRESS [[__no_unique_address__]]
 #  else
-#    define _LIBCPP_NO_UNIQUE_ADDRESS /* nothing */
-// Note that this can be replaced by #error as soon as clang-cl
-// implements msvc::no_unique_address, since there should be no C++20
-// compiler that doesn't support one of the two attributes at that point.
-// We generally don't want to use this macro outside of C++20-only code,
-// because using it conditionally in one language version only would make
-// the ABI inconsistent.
+#    define _LIBCPP_NO_UNIQUE_ADDRESS [[__no_unique_address__]]
 #  endif
 
 // c8rtomb() and mbrtoc8() were added in C++20 and C23. Support for these
diff --git a/libcxx/include/__cxx03/__type_traits/datasizeof.h b/libcxx/include/__cxx03/__type_traits/datasizeof.h
index d428856d13a66..c207f59ac2cc0 100644
--- a/libcxx/include/__cxx03/__type_traits/datasizeof.h
+++ b/libcxx/include/__cxx03/__type_traits/datasizeof.h
@@ -26,29 +26,16 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_keyword(__datasizeof) || __has_extension(datasizeof)
+// TODO: Enable this again once #94816 is fixed.
+#if (__has_keyword(__datasizeof) || __has_extension(datasizeof)) && 0
 template <class _Tp>
 inline const size_t __datasizeof_v = __datasizeof(_Tp);
 #else
-// NOLINTNEXTLINE(readability-redundant-preprocessor) This is https://llvm.org/PR64825
-#  if __has_cpp_attribute(__no_unique_address__)
 template <class _Tp>
 struct _FirstPaddingByte {
-  [[__no_unique_address__]] _Tp __v_;
+  _LIBCPP_NO_UNIQUE_ADDRESS _Tp __v_;
   char __first_padding_byte_;
 };
-#  else
-template <class _Tp, bool = __libcpp_is_final<_Tp>::value || !is_class<_Tp>::value>
-struct _FirstPaddingByte : _Tp {
-  char __first_padding_byte_;
-};
-
-template <class _Tp>
-struct _FirstPaddingByte<_Tp, true> {
-  _Tp __v_;
-  char __first_padding_byte_;
-};
-#  endif // __has_cpp_attribute(__no_unique_address__)
 
 // _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow
 // the use as an extension.
diff --git a/libcxx/test/libcxx-03/type_traits/datasizeof.compile.pass.cpp b/libcxx/test/libcxx-03/type_traits/datasizeof.compile.pass.cpp
index 67a4dccb0f5fe..699c5101bb7fe 100644
--- a/libcxx/test/libcxx-03/type_traits/datasizeof.compile.pass.cpp
+++ b/libcxx/test/libcxx-03/type_traits/datasizeof.compile.pass.cpp
@@ -6,9 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
-#include <__type_traits/datasizeof.h>
+#include <__cxx03/__type_traits/datasizeof.h>
 #include <cstdint>
 #include <type_traits>