[clang] [clang] CTAD: use index and depth to retrieve template parameter for TemplateParamsReferencedInTemplateArgumentList (PR #98013)

[Matheus Izvekov via cfe-commits]

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@@ -2653,20 +2653,34 @@ struct ConvertConstructorToDeductionGuideTransform {
 // Find all template parameters that appear in the given DeducedArgs.
 // Return the indices of the template parameters in the TemplateParams.
 SmallVector<unsigned> TemplateParamsReferencedInTemplateArgumentList(
-    ArrayRef<NamedDecl *> TemplateParams,
+    const TemplateParameterList* TemplateParamsList,
     ArrayRef<TemplateArgument> DeducedArgs) {
   struct TemplateParamsReferencedFinder
       : public RecursiveASTVisitor<TemplateParamsReferencedFinder> {
+    const TemplateParameterList* TemplateParamList;
     llvm::DenseSet<NamedDecl *> TemplateParams;
     llvm::DenseSet<const NamedDecl *> ReferencedTemplateParams;
 
-    TemplateParamsReferencedFinder(ArrayRef<NamedDecl *> TemplateParams)
-        : TemplateParams(TemplateParams.begin(), TemplateParams.end()) {}
+    TemplateParamsReferencedFinder(
+        const TemplateParameterList *TemplateParamList)
+        : TemplateParamList(TemplateParamList),
+          TemplateParams(TemplateParamList->begin(), TemplateParamList->end()) {
+    }
 
     bool VisitTemplateTypeParmType(TemplateTypeParmType *TTP) {
-      MarkAppeared(TTP->getDecl());
+      // We use the index and depth to retrieve the corresponding template
+      // parameter from the parameter list.
+      // Note that Clang may not preserve type sugar during template argument
+      // deduction. In such cases, the TTP is a canonical TemplateTypeParamType,
+      // which only retains its index and depth information.
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mizvekov wrote:

I would remove this section of the comment, as I think this sort of implies it would be ok to use the type sugar if it were available, but if we rely on this for correct compilation of the program, it doesn't make sense to call it type sugar anymore.
```suggestion
```

https://github.com/llvm/llvm-project/pull/98013