llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Utkarsh Saxena (usx95)
<details>
<summary>Changes</summary>
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
Fixes https://github.com/llvm/llvm-project/issues/107560
---
Full diff: https://github.com/llvm/llvm-project/pull/108197.diff
1 Files Affected:
- (modified) clang/lib/Sema/TreeTransform.h (+2-1)
``````````diff
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 0daf620b4123e4..0de43d2127b12f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4361,7 +4361,8 @@ 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())
return true;
``````````
</details>
https://github.com/llvm/llvm-project/pull/108197
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