[clang] [Clang] Implement C++26’s P2893R3 ‘Variadic friends’ (PR #101448)

[via cfe-commits]

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@@ -17354,12 +17354,18 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
 DeclResult Sema::ActOnTemplatedFriendTag(
     Scope *S, SourceLocation FriendLoc, unsigned TagSpec, SourceLocation TagLoc,
     CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
-    const ParsedAttributesView &Attr, MultiTemplateParamsArg TempParamLists) {
+    SourceLocation EllipsisLoc, const ParsedAttributesView &Attr,
+    MultiTemplateParamsArg TempParamLists) {
   TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
 
   bool IsMemberSpecialization = false;
   bool Invalid = false;
 
+  // FIXME: This works for now; revisit once we support packs in NNSs.
+  if (EllipsisLoc.isValid())
+    Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
+        << SourceRange(FriendLoc, NameLoc);
+
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cor3ntin wrote:

roughly: collect unexpanded packs. Match them to their declaration and see if that declaration is one if the template parameter of the current declaration

https://github.com/llvm/llvm-project/pull/101448