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

[via cfe-commits]

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@@ -17509,8 +17528,17 @@ Decl *Sema::ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
   if (TheDeclarator.isInvalidType())
     return nullptr;
 
-  if (DiagnoseUnexpandedParameterPack(Loc, TSI, UPPC_FriendDeclaration))
+  // If '...' is present, the type must contain an unexpanded parameter
+  // pack, and vice versa.
+  if (EllipsisLoc.isInvalid() &&
+      DiagnoseUnexpandedParameterPack(Loc, TSI, UPPC_FriendDeclaration))
     return nullptr;
+  if (EllipsisLoc.isValid() &&
+      !TSI->getType()->containsUnexpandedParameterPack()) {
+    Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
+        << TSI->getTypeLoc().getSourceRange();
+    EllipsisLoc = SourceLocation();
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cor3ntin wrote:

I think this is the only outstanding nit, and then we can merge

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