[clang] [Sema] Default arguments for template parameters affect ContainsUnexpandedPacks (PR #99880)

[via cfe-commits]

================
@@ -61,27 +61,43 @@ TemplateParameterList::TemplateParameterList(const 
ASTContext& C,
 
     bool IsPack = P->isTemplateParameterPack();
     if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
-      if (!IsPack && NTTP->getType()->containsUnexpandedParameterPack())
-        ContainsUnexpandedParameterPack = true;
+      if (!IsPack) {
+        if (NTTP->getType()->containsUnexpandedParameterPack())
+          ContainsUnexpandedParameterPack = true;
+        else if (NTTP->hasDefaultArgument() &&
+                 NTTP->getDefaultArgument()
+                     .getArgument()
+                     .containsUnexpandedParameterPack())
+          ContainsUnexpandedParameterPack = true;
+      }
       if (NTTP->hasPlaceholderTypeConstraint())
         HasConstrainedParameters = true;
     } else if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P)) {
-      if (!IsPack &&
-          TTP->getTemplateParameters()->containsUnexpandedParameterPack())
-        ContainsUnexpandedParameterPack = true;
-    } else if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
-      if (const TypeConstraint *TC = TTP->getTypeConstraint()) {
-        if (TC->getImmediatelyDeclaredConstraint()
-            ->containsUnexpandedParameterPack())
+      if (!IsPack) {
+        if (TTP->getTemplateParameters()->containsUnexpandedParameterPack())
           ContainsUnexpandedParameterPack = true;
+        else if (TTP->hasDefaultArgument() &&
+                 TTP->getDefaultArgument()
+                     .getArgument()
+                     .containsUnexpandedParameterPack())
+          ContainsUnexpandedParameterPack = true;
+      }
+    } else if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
+      if (!IsPack && TTP->hasDefaultArgument() &&
+          TTP->getDefaultArgument()
+              .getArgument()
+              .containsUnexpandedParameterPack()) {
+        ContainsUnexpandedParameterPack = true;
+      } else if (const TypeConstraint *TC = TTP->getTypeConstraint();
+                 TC && TC->getImmediatelyDeclaredConstraint()
+                           ->containsUnexpandedParameterPack()) {
+        ContainsUnexpandedParameterPack = true;
----------------
cor3ntin wrote:

Do we properly handle template template parameters?

```cpp
template <template <typename> typename...T>
void f() {
    (([]<template <typename> typename U = T> {}(), T()), ...);
};
```

https://github.com/llvm/llvm-project/pull/99880
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