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

[Ilya Biryukov 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;
----------------
ilya-biryukov wrote:

This code on line 84 does the right thing, assuming the `TempateArgument` 
returns correct values for `containsUnexpandedParameterPack()`.

If `TemplateArgument` is wrong, I would argue it's a problem for another patch, 
I'll make sure we test this when fixing the fold expressions (I don't know any 
other way to notice if we're getting the result wrong here).

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