[clang] [libcxx] [libc++] Remove potential 0-sized array in __compressed_pair_padding (PR #109028)

[via cfe-commits]

https://github.com/serge-sans-paille updated https://github.com/llvm/llvm-project/pull/109028

>From fc06c00eb299b070106dcffd7d13f3a11eb4c1f0 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Wed, 18 Sep 2024 08:31:04 +0200
Subject: [PATCH 1/2] Consistent strict flex array

---
 clang/lib/Sema/SemaDecl.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 31bf50a32a83c3..b92973299f9c13 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -18931,7 +18931,11 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
       FD->setInvalidDecl();
       EnclosingDecl->setInvalidDecl();
       continue;
-    } else if (FDTy->isIncompleteArrayType() &&
+    } else if (Decl::isFlexibleArrayMemberLike(
+                   Context, FD, FD->getType(),
+                   LangOptions::StrictFlexArraysLevelKind::ZeroOrIncomplete
+                   /*getLangOpts().getStrictFlexArraysLevel()*/,
+                   true) &&
                (Record || isa<ObjCContainerDecl>(EnclosingDecl))) {
       if (Record) {
         // Flexible array member.

>From 0c70565390aa326fb9aac4fda77280f45c1837df Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Tue, 17 Sep 2024 20:39:05 +0200
Subject: [PATCH 2/2] [libc++] Remove potential 0-sized array in
 __compressed_pair_padding

---
 libcxx/include/__memory/compressed_pair.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/__memory/compressed_pair.h b/libcxx/include/__memory/compressed_pair.h
index 629e3ad8848ffa..77928af434f1f4 100644
--- a/libcxx/include/__memory/compressed_pair.h
+++ b/libcxx/include/__memory/compressed_pair.h
@@ -52,13 +52,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #ifndef _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
 
-template <class _ToPad>
+template <class _ToPad,
+          bool _Empty = ((is_empty<_ToPad>::value && !__libcpp_is_final<_ToPad>::value) ||
+                         is_reference<_ToPad>::value || sizeof(_ToPad) - __datasizeof_v<_ToPad> == 0)>
 class __compressed_pair_padding {
-  char __padding_[((is_empty<_ToPad>::value && !__libcpp_is_final<_ToPad>::value) || is_reference<_ToPad>::value)
-                      ? 0
-                      : sizeof(_ToPad) - __datasizeof_v<_ToPad>];
+  char __padding_[sizeof(_ToPad) - __datasizeof_v<_ToPad>];
 };
 
+template <class _ToPad>
+class __compressed_pair_padding<_ToPad, true> {};
+
 #  define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2)                                                  \
     _LIBCPP_NO_UNIQUE_ADDRESS __attribute__((__aligned__(_LIBCPP_ALIGNOF(T2)))) T1 Initializer1;                       \
     _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);          \