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

Summary:
This addresses the FIXME in the code. There will be tests for the new
behavior in an upcoming #86265, which also addresses other bugs that
prevent exposing the wrong results of `ContainsUnexpandedPacks` in the
outputs of the compiler without crashes.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251126

Author: ilya-biryukov

clang/lib/AST/DeclTemplate.cpp

@@ -44,6 +44,12 @@ using namespace clang;
 // TemplateParameterList Implementation
 //===----------------------------------------------------------------------===//
 
+template <class TemplateParam>
+static bool
+DefaultTemplateArgumentContainsUnexpandedPack(const TemplateParam &P) {
+  return P.hasDefaultArgument() &&
+         P.getDefaultArgument().getArgument().containsUnexpandedParameterPack();
+}
 
 TemplateParameterList::TemplateParameterList(const ASTContext& C,
                                              SourceLocation TemplateLoc,
@@ -61,27 +67,30 @@ TemplateParameterList::TemplateParameterList(const ASTContext& C,
 
     bool IsPack = P->isTemplateParameterPack();
     if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
-      if (!IsPack && NTTP->getType()->containsUnexpandedParameterPack())
+      if (!IsPack && (NTTP->getType()->containsUnexpandedParameterPack() ||
+                      DefaultTemplateArgumentContainsUnexpandedPack(*NTTP)))
         ContainsUnexpandedParameterPack = true;
       if (NTTP->hasPlaceholderTypeConstraint())
         HasConstrainedParameters = true;
     } else if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(P)) {
       if (!IsPack &&
-          TTP->getTemplateParameters()->containsUnexpandedParameterPack())
+          (TTP->getTemplateParameters()->containsUnexpandedParameterPack() ||
+           DefaultTemplateArgumentContainsUnexpandedPack(*TTP))) {
         ContainsUnexpandedParameterPack = true;
+      }
     } else if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
-      if (const TypeConstraint *TC = TTP->getTypeConstraint()) {
-        if (TC->getImmediatelyDeclaredConstraint()
-            ->containsUnexpandedParameterPack())
-          ContainsUnexpandedParameterPack = true;
+      if (!IsPack && DefaultTemplateArgumentContainsUnexpandedPack(*TTP)) {
+        ContainsUnexpandedParameterPack = true;
+      } else if (const TypeConstraint *TC = TTP->getTypeConstraint();
+                 TC && TC->getImmediatelyDeclaredConstraint()
+                           ->containsUnexpandedParameterPack()) {
+        ContainsUnexpandedParameterPack = true;
       }
       if (TTP->hasTypeConstraint())
         HasConstrainedParameters = true;
     } else {
       llvm_unreachable("unexpected template parameter type");
     }
-    // FIXME: If a default argument contains an unexpanded parameter pack, the
-    // template parameter list does too.
   }
 
   if (HasRequiresClause) {