[clang] [Clang] Reset ArgPackSubstIndex before rewriting CTAD template parameters (PR #141741)

[Erich Keane via cfe-commits]

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@@ -1099,6 +1099,10 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
   // parameters, used for building `TemplateArgsForBuildingFPrime`.
   SmallVector<TemplateArgument, 16> TransformedDeducedAliasArgs(
       AliasTemplate->getTemplateParameters()->size());
+  // We might be already within a pack expansion, but rewriting template
+  // parameters is independent of that. (We may or may not expand new packs
+  // when rewriting. So clear the state)
+  Sema::ArgPackSubstIndexRAII _(SemaRef, std::nullopt);
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erichkeane wrote:

```suggestion
  Sema::ArgPackSubstIndexRAII PackSubstReset(SemaRef, std::nullopt);
```

https://github.com/llvm/llvm-project/pull/141741