[clang] Persist Attr::IsPackExpansion into the PCH

[nridge]

Summary

Fixes https://github.com/clangd/clangd/issues/309

[kadircet]

oh wow! thanks for looking into this.

Looking at the history, it seems like an oversight. The patch that introduced packexpansion bit: https://github.com/llvm/llvm-project/commit/44c247f0f009eec70a193335c8a353d6f8602bfd.

it didn't add that field to pchattr serialization code unfortunately. (another example on isimplicit, which adds attr to record https://github.com/llvm/llvm-project/commit/36a5350e51a6dcfae424332aaa266a1e5bfb8c4f)

Could you add tests though ? Something similar to clang/test/PCH/attrs-PR8406.c should be enough.

[kadircet]

also please update the commit message to mention "PCH" rather than "serialized ast".

[nridge]

Add test

[kadircet]

thanks LGTM!

[aaron.ballman]

LGTM!

[sammccall]

@rsmith are you OK with merging this into the release branch for 10.0.1?
cc @tstellar

[rsmith]

In D77194#2086245, @sammccall wrote:

@rsmith are you OK with merging this into the release branch for 10.0.1?
cc @tstellar

Yes, please do.

(I would like for us to eventually maintain AST file format compatibility between patch releases, which this change would break, but as far as I'm aware the status quo is still that we include the version number and revision as part of the signature of the AST file and will reject the file if it differs even by clang patch release, so the compatibility there is only a future-facing concern, not one that affects this change.)

clang/test/PCH/cxx-attrs-packexpansion.cpp

+// Test this without pch.
+// RUN: %clang_cc1 -include %s -emit-llvm -o - %s
+
+// Test with pch.
+// RUN: %clang_cc1 -emit-pch -o %t %s
+// RUN: %clang_cc1 -include-pch %t -emit-llvm -o - %s
+
+#ifndef HEADER
+#define HEADER
+
+template<typename T, typename... Types>
+struct static_variant {
+    alignas(Types...) T storage[10];
+};
+
+#else
+
+struct A {
+    static_variant<int> a;
+};
+struct B {
+    static_variant<A> _b;
+};
+
+#endif

clang/utils/TableGen/ClangAttrEmitter.cpp

for (const auto *Attr : Attrs) {
     const Record &R = *Attr;
     if (!R.getValueAsBit("ASTNode"))
       continue;
 
     OS << "  case attr::" << R.getName() << ": {\n";
     if (R.isSubClassOf(InhClass))
       OS << "    bool isInherited = Record.readInt();\n";
     OS << "    bool isImplicit = Record.readInt();\n";
+    OS << "    bool isPackExpansion = Record.readInt();\n";
     ArgRecords = R.getValueAsListOfDefs("Args");
     Args.clear();
     for (const auto *Arg : ArgRecords) {
       Args.emplace_back(createArgument(*Arg, R.getName()));
       Args.back()->writePCHReadDecls(OS);
     }
     OS << "    New = new (Context) " << R.getName() << "Attr(Context, Info";
     for (auto const &ri : Args) {
       OS << ", ";
       ri->writePCHReadArgs(OS);
     }
     OS << ");\n";
     if (R.isSubClassOf(InhClass))
       OS << "    cast<InheritableAttr>(New)->setInherited(isInherited);\n";
     OS << "    New->setImplicit(isImplicit);\n";
+    OS << "    New->setPackExpansion(isPackExpansion);\n";
     OS << "    break;\n";
     OS << "  }\n";
   }
   OS << "  }\n";
 }
 
 // Emits the code to write an attribute to a precompiled header.
 void EmitClangAttrPCHWrite(RecordKeeper &Records, raw_ostream &OS) {
for (const auto *Attr : Attrs) {
     OS << "  case attr::" << R.getName() << ": {\n";
     Args = R.getValueAsListOfDefs("Args");
     if (R.isSubClassOf(InhClass) || !Args.empty())
       OS << "    const auto *SA = cast<" << R.getName()
          << "Attr>(A);\n";
     if (R.isSubClassOf(InhClass))
       OS << "    Record.push_back(SA->isInherited());\n";
     OS << "    Record.push_back(A->isImplicit());\n";
+    OS << "    Record.push_back(A->isPackExpansion());\n";
 
     for (const auto *Arg : Args)
       createArgument(*Arg, R.getName())->writePCHWrite(OS);
     OS << "    break;\n";
     OS << "  }\n";
   }
   OS << "  }\n";
 }