[clang] [Clang][Sema] Refactor collection of multi-level template argument lists (PR #106585)

[Younan Zhang via cfe-commits]

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@@ -4682,6 +4683,17 @@ bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                       ParmVarDecl *Param) {
   assert(Param->hasUninstantiatedDefaultArg());
 
+  NamedDecl *Pattern = FD;
+  std::optional<ArrayRef<TemplateArgument>> Innermost;
+
+  if (FD->isCXXClassMember() &&
+      !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(FD)) {
+    if (FunctionTemplateDecl *FTD = FD->getPrimaryTemplate()) {
+      Pattern = FTD->getFirstDecl();
+      Innermost = FD->getTemplateSpecializationArgs()->asArray();
+    }
+  }
+
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zyn0217 wrote:

Yeah, this is why default arguments are special: the default arguments are copied from its first declaration and thus could reference outer template parameters outside of the specialization. Can you add some comments around it?

(I added the `ForDefaultArgumentSubstitution` flag because I somehow don't like special-casing something that is possibly handled by `getTemplateInstantiationArgs`, though that would add one more state to that function. So this is a trade-off.)

https://github.com/llvm/llvm-project/pull/106585