[clang] [clang] fix serialization for SubstNonTypeTemplateParmPackExpr (PR #135428)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov created https://github.com/llvm/llvm-project/pull/135428

This fixes a PCM non-determinism regression reported here: https://github.com/llvm/llvm-project/pull/134560#issuecomment-2797744370

There was a bit in `SubstNonTypeTemplateParmPackExpr` which we missed to serialize, and that bit eventually propagates to `SubstNonTypeTemplateParmExpr`.

As a drive by, improve serialization for PackIndex on SubstNonTypeTemplateParmExpr by using the newly introduced UnsignedOrNone helpers.

There are no release notes since this regression was never released.

>From 72364b18f0cfc44219d73f648618eb4a190b4a3d Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Fri, 11 Apr 2025 16:47:55 -0300
Subject: [PATCH] [clang] fix serialization for
 SubstNonTypeTemplateParmPackExpr

This fixes a PCM non-determinism regression reported here:
https://github.com/llvm/llvm-project/pull/134560#issuecomment-2797744370

There was a bit in `SubstNonTypeTemplateParmPackExpr` which we missed
to serialize, and that bit eventually propagates to
`SubstNonTypeTemplateParmExpr`.

As a drive by, improve serialization for PackIndex on SubstNonTypeTemplateParmExpr
by using the newly introduced UnsignedOrNone helpers.

There are no release notes since this regression was never released.
---
 clang/lib/Serialization/ASTReaderStmt.cpp | 6 ++----
 clang/lib/Serialization/ASTWriterStmt.cpp | 5 ++---
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index 715aaf6452264..f41cfcc53a35d 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -2226,10 +2226,7 @@ void ASTStmtReader::VisitSubstNonTypeTemplateParmExpr(
   E->AssociatedDeclAndRef.setPointer(readDeclAs<Decl>());
   E->AssociatedDeclAndRef.setInt(CurrentUnpackingBits->getNextBit());
   E->Index = CurrentUnpackingBits->getNextBits(/*Width=*/12);
-  if (CurrentUnpackingBits->getNextBit())
-    E->PackIndex = Record.readInt();
-  else
-    E->PackIndex = 0;
+  E->PackIndex = Record.readUnsignedOrNone().toInternalRepresentation();
   E->Final = CurrentUnpackingBits->getNextBit();
   E->SubstNonTypeTemplateParmExprBits.NameLoc = readSourceLocation();
   E->Replacement = Record.readSubExpr();
@@ -2239,6 +2236,7 @@ void ASTStmtReader::VisitSubstNonTypeTemplateParmPackExpr(
                                           SubstNonTypeTemplateParmPackExpr *E) {
   VisitExpr(E);
   E->AssociatedDecl = readDeclAs<Decl>();
+  E->Final = CurrentUnpackingBits->getNextBit();
   E->Index = Record.readInt();
   TemplateArgument ArgPack = Record.readTemplateArgument();
   if (ArgPack.getKind() != TemplateArgument::Pack)
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index 939b37c547349..b9eabd5ddb64c 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -2228,9 +2228,7 @@ void ASTStmtWriter::VisitSubstNonTypeTemplateParmExpr(
   Record.AddDeclRef(E->getAssociatedDecl());
   CurrentPackingBits.addBit(E->isReferenceParameter());
   CurrentPackingBits.addBits(E->getIndex(), /*Width=*/12);
-  CurrentPackingBits.addBit((bool)E->getPackIndex());
-  if (auto PackIndex = E->getPackIndex())
-    Record.push_back(*PackIndex + 1);
+  Record.writeUnsignedOrNone(E->getPackIndex());
   CurrentPackingBits.addBit(E->getFinal());
 
   Record.AddSourceLocation(E->getNameLoc());
@@ -2242,6 +2240,7 @@ void ASTStmtWriter::VisitSubstNonTypeTemplateParmPackExpr(
                                           SubstNonTypeTemplateParmPackExpr *E) {
   VisitExpr(E);
   Record.AddDeclRef(E->getAssociatedDecl());
+  CurrentPackingBits.addBit(E->getFinal());
   Record.push_back(E->getIndex());
   Record.AddTemplateArgument(E->getArgumentPack());
   Record.AddSourceLocation(E->getParameterPackLocation());