[clang] [Clang] Remove the wrong assumption when rebuilding SizeOfPackExprs for constraint normalization (PR #115120)

[Matheus Izvekov via cfe-commits]

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@@ -1881,6 +1871,15 @@ Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
       TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
 
       if (TTP->isParameterPack()) {
+        // We might not have an index for pack expansion when normalizing
+        // constraint expressions. In that case, resort to instantiation scopes
+        // for the transformed declarations.
+        if (SemaRef.ArgumentPackSubstitutionIndex == -1 &&
+            SemaRef.CodeSynthesisContexts.back().Kind ==
+                Sema::CodeSynthesisContext::ConstraintNormalization) {
+          return SemaRef.FindInstantiatedDecl(Loc, cast<NamedDecl>(D),
+                                              TemplateArgs);
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mizvekov wrote:

Why don't we have an index in these cases? Is this something we could fix in the future?

https://github.com/llvm/llvm-project/pull/115120