[clang] Allow packing fields into tail padding of record fields (PR #122197)

[Reid Kleckner via cfe-commits]

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@@ -803,10 +804,16 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
       if (!AppendField(Field, Layout.getFieldOffset(FieldNo), EltInit,
                        AllowOverwrite))
         return false;
-      // After emitting a non-empty field with [[no_unique_address]], we may
-      // need to overwrite its tail padding.
-      if (Field->hasAttr<NoUniqueAddressAttr>())
-        AllowOverwrite = true;
+
+      // Allow overwrites after a field with tail padding. This allows
+      // overwriting tail padding of fields carrying [[no_unique_address]]
+      // without checking for it, since it is not necessarily present in debug
+      // info.
+      if (const CXXRecordDecl *FieldRD = FieldTy->getAsCXXRecordDecl()) {
+        const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(FieldRD);
+        if (Layout.getDataSize() < Layout.getSize())
+          AllowOverwrite = true;
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rnk wrote:

I hadn't caught the other one! I only happened to find this one through testing, not inspection. I'll have to figure out how to exercise the other codepath.

https://github.com/llvm/llvm-project/pull/122197