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

[Younan Zhang via cfe-commits]

================
@@ -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));
+  }
+
----------------
zyn0217 wrote:

@mizvekov I looked into `ConvertDeducedTemplateArguments` and I'm afraid it won't help

In deduction guide synthesizing, we have a unique template parameter transform that does handle default parameters (which also covers non-alias deduction guide synthesizing) so when we find a parameter (e.g. `U` in example) used by the RHS (`A<U>`), we form a new TTP on top of the TTP `U = T`. And, as `T` is not referenced by `A<U>`, the declaration of which won't be created anyway, so we ended up with a problematic `template <class U = <template-param-0-0>>` in the eventual CTAD guide.

```cpp
template <class _Ty> struct A {};

template <class T = int, class U = T>
using AA = A<U>;
```

This is where the problem lies. And with `ConvertDeducedTemplateArguments`, which would substitute the default arguments with the deduced ones in non-rewrite mode, we end up building Subst* nodes that we never want in CTAD.

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