[clang] [clang] Do not substitute parameter pack while retaining the pack expansion (PR #108197)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 created https://github.com/llvm/llvm-project/pull/108197

Consider when Input[I] is a VarDecl with parameter pack. We would have already expanded the pack before the code change in the loop`for (unsigned I = 0; I != *NumExpansions; ++I) {`.

Now in `if (RetainExpansion) {`, without this change, we continue to substitute the pack in the pattern even when we do not have meaningful `ArgumentPackSubstitutionIndex` set. 

This leads to use of an invalid pack substitution index in `TemplateInstantiator::TransformFunctionParmPackRefExpr` containing `TransformedDecl = (*Pack)[getSema().ArgumentPackSubstitutionIndex];`

This change set `ArgumentPackSubstitutionIndex` to `-1` while retaining expansion to instruct `TransformFunctionParmPackRefExpr` to build `FunctionParmPackExpr` instead of substituting the param pack.

---

There are other instances of `RetainExpansion` and IIUC, they should also unset the `ArgumentPackSubstitutionIndex`. It would be great if someone can verify my understanding. If this is correct then we could instead have a `ArgumentPackSubstitutionIndexRAII` as part of `ForgetPartiallySubstitutedPackRAII`.


Fixes https://github.com/llvm/llvm-project/issues/63819

>From 5901d82ea0543074853b963f7dc9106a6fe3bcee Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 11 Sep 2024 11:33:45 +0000
Subject: [PATCH 1/2] [clang] Do not expand pack while retaining expansion

---
 clang/lib/Sema/TreeTransform.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 0daf620b4123e4..a40673b04764da 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4361,7 +4361,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr *const *Inputs,
       // forgetting the partially-substituted parameter pack.
       if (RetainExpansion) {
         ForgetPartiallySubstitutedPackRAII Forget(getDerived());
-
+        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
         ExprResult Out = getDerived().TransformExpr(Pattern);
         if (Out.isInvalid())
           return true;

>From 97fbf34c3edd09348fb48b4dc66f1d854516e8ef Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 11 Sep 2024 11:59:58 +0000
Subject: [PATCH 2/2] Add comment

---
 clang/lib/Sema/TreeTransform.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index a40673b04764da..0de43d2127b12f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4361,6 +4361,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr *const *Inputs,
       // forgetting the partially-substituted parameter pack.
       if (RetainExpansion) {
         ForgetPartiallySubstitutedPackRAII Forget(getDerived());
+        // Simple transform producing another pack expansion.
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
         ExprResult Out = getDerived().TransformExpr(Pattern);
         if (Out.isInvalid())