[clang] [Clang] Handle default template arguments for alias CTAD guides (PR #134807)

[Younan Zhang via cfe-commits]

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@@ -690,6 +690,23 @@ SmallVector<unsigned> TemplateParamsReferencedInTemplateArgumentList(
   SemaRef.MarkUsedTemplateParameters(
       DeducedArgs, TemplateParamsList->getDepth(), ReferencedTemplateParams);
 
+  auto MarkDefaultArgs = [&](auto *Param) {
+    if (!Param || !Param->hasDefaultArgument())
+      return;
+    SemaRef.MarkUsedTemplateParameters(
+        Param->getDefaultArgument().getArgument(),
+        TemplateParamsList->getDepth(), ReferencedTemplateParams);
+  };
+
+  for (unsigned Index = 0; Index < TemplateParamsList->size(); ++Index) {
+    if (!ReferencedTemplateParams[Index])
+      continue;
+    auto *Param = TemplateParamsList->getParam(Index);
+    MarkDefaultArgs(dyn_cast<TemplateTypeParmDecl>(Param));
+    MarkDefaultArgs(dyn_cast<NonTypeTemplateParmDecl>(Param));
+    MarkDefaultArgs(dyn_cast<TemplateTemplateParmDecl>(Param));
+  }
+
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zyn0217 wrote:

> I wonder if we are missing a step to get the converted template argument list, and should be calling this with that instead of the deduced arguments directly.

The fact that makes me think it sensible is, in the later template parameter transform, we do retain/tranaform the default arguments. That means, the transformed template parameters which has default arguments could end up referring to anything that won't otherwise appear in the synthesized template parameter list. And this is exactly where the issue raised.

Re the ConvertDeducedTemplateArguments, i think we probably want it, but I couldn't immediately recall what job it does to handle default arguments.

https://github.com/llvm/llvm-project/pull/134807