[all-commits] [llvm/llvm-project] 849d1b: [clang] Do not substitute parameter pack while ret...

[Utkarsh Saxena via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 849d1b8b1f1fc16dc28b07da358515a52b79ea81
      https://github.com/llvm/llvm-project/commit/849d1b8b1f1fc16dc28b07da358515a52b79ea81
  Author: Utkarsh Saxena <usx at google.com>
  Date:   2024-09-12 (Thu, 12 Sep 2024)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/lib/Sema/TreeTransform.h
    M clang/test/SemaTemplate/pack-deduction.cpp

  Log Message:
  -----------
  [clang] Do not substitute parameter pack while retaining the pack expansion (#108197)

(In reference to https://github.com/llvm/llvm-project/pull/108197/commits/5901d82ea0543074853b963f7dc9106a6fe3bcee)
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` in
`TransformedDecl = (*Pack)[getSema().ArgumentPackSubstitutionIndex];`

This change sets `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`.

EDIT: I have moved this to `ForgetPartiallySubstitutedPackRAII`.

Fixes https://github.com/llvm/llvm-project/issues/63819
Fixes https://github.com/llvm/llvm-project/issues/107560



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