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

[Aaron Ballman via cfe-commits]

================
@@ -1442,8 +1442,49 @@ Decl *TemplateDeclInstantiator::VisitFriendDecl(FriendDecl *D) {
     if (D->isUnsupportedFriend()) {
       InstTy = Ty;
     } else {
-      InstTy = SemaRef.SubstType(Ty, TemplateArgs,
-                                 D->getLocation(), DeclarationName());
+      if (D->isVariadic()) {
+        SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+        SemaRef.collectUnexpandedParameterPacks(Ty->getTypeLoc(), Unexpanded);
+        assert(!Unexpanded.empty() && "Pack expansion without packs");
+
+        bool ShouldExpand = true;
+        bool RetainExpansion = false;
+        std::optional<unsigned> NumExpansions;
+        if (SemaRef.CheckParameterPacksForExpansion(
+                D->getEllipsisLoc(), D->getSourceRange(), Unexpanded,
+                TemplateArgs, ShouldExpand, RetainExpansion, NumExpansions))
+          return nullptr;
+
+        assert(!RetainExpansion &&
+               "should never retain an expansion for a FriendPackDecl");
+
+        if (ShouldExpand) {
+          SmallVector<FriendDecl *> Decls;
+          for (unsigned I = 0; I != *NumExpansions; I++) {
+            Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, I);
+            TypeSourceInfo *TSI = SemaRef.SubstType(
+                Ty, TemplateArgs, D->getEllipsisLoc(), DeclarationName());
+            if (!TSI)
+              return nullptr;
+
+            auto FD =
+                FriendDecl::Create(SemaRef.Context, Owner, D->getLocation(),
+                                   TSI, D->getFriendLoc());
+
+            FD->setAccess(AS_public);
+            Owner->addDecl(FD);
----------------
AaronBallman wrote:

I *think* that is not a bug (I think this gets called on the thing that is being friended, not on the friend itself), but I'd appreciate a second opinion from @zygoloid or @rjmccall 

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