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

Implement P2893R3 ‘Variadic friends’ for C++26.

This closes #98587.

Co-authored-by: Younan Zhang <zyn7109@gmail.com>

Author: Sirraide

clang/docs/LanguageExtensions.rst

@@ -1505,6 +1505,7 @@ Attributes on Lambda-Expressions                                              C+
 Attributes on Structured Bindings            __cpp_structured_bindings        C++26         C++03
 Pack Indexing                                __cpp_pack_indexing              C++26         C++03
 ``= delete ("should have a reason");``       __cpp_deleted_function           C++26         C++03
+Variadic Friends                             __cpp_variadic_friend            C++26         C++03
 -------------------------------------------- -------------------------------- ------------- -------------
 Designated initializers (N494)                                                C99           C89
 Array & element qualification (N2607)                                         C23           C89

clang/docs/ReleaseNotes.rst

@@ -126,6 +126,8 @@ C++2c Feature Support
 - Add ``__builtin_is_virtual_base_of`` intrinsic, which supports
   `P2985R0 A type trait for detecting virtual base classes <https://wg21.link/p2985r0>`_
 
+- Implemented `P2893R3 Variadic Friends <https://wg21.link/P2893>`_
+
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/DeclFriend.h

@@ -70,6 +70,9 @@ class FriendDecl final
   // Location of the 'friend' specifier.
   SourceLocation FriendLoc;
 
+  // Location of the '...', if present.
+  SourceLocation EllipsisLoc;
+
   /// True if this 'friend' declaration is unsupported.  Eventually we
   /// will support every possible friend declaration, but for now we
   /// silently ignore some and set this flag to authorize all access.
@@ -82,10 +85,11 @@ class FriendDecl final
   unsigned NumTPLists : 31;
 
   FriendDecl(DeclContext *DC, SourceLocation L, FriendUnion Friend,
-             SourceLocation FriendL,
+             SourceLocation FriendL, SourceLocation EllipsisLoc,
              ArrayRef<TemplateParameterList *> FriendTypeTPLists)
       : Decl(Decl::Friend, DC, L), Friend(Friend), FriendLoc(FriendL),
-        UnsupportedFriend(false), NumTPLists(FriendTypeTPLists.size()) {
+        EllipsisLoc(EllipsisLoc), UnsupportedFriend(false),
+        NumTPLists(FriendTypeTPLists.size()) {
     for (unsigned i = 0; i < NumTPLists; ++i)
       getTrailingObjects<TemplateParameterList *>()[i] = FriendTypeTPLists[i];
   }
@@ -110,7 +114,7 @@ class FriendDecl final
 
   static FriendDecl *
   Create(ASTContext &C, DeclContext *DC, SourceLocation L, FriendUnion Friend_,
-         SourceLocation FriendL,
+         SourceLocation FriendL, SourceLocation EllipsisLoc = {},
          ArrayRef<TemplateParameterList *> FriendTypeTPLists = std::nullopt);
   static FriendDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID,
                                         unsigned FriendTypeNumTPLists);
@@ -143,8 +147,24 @@ class FriendDecl final
     return FriendLoc;
   }
 
+  /// Retrieves the location of the '...', if present.
+  SourceLocation getEllipsisLoc() const { return EllipsisLoc; }
+
   /// Retrieves the source range for the friend declaration.
   SourceRange getSourceRange() const override LLVM_READONLY {
+    if (TypeSourceInfo *TInfo = getFriendType()) {
+      SourceLocation StartL =
+          (NumTPLists == 0) ? getFriendLoc()
+                            : getTrailingObjects<TemplateParameterList *>()[0]
+                                  ->getTemplateLoc();
+      SourceLocation EndL = isPackExpansion() ? getEllipsisLoc()
+                                              : TInfo->getTypeLoc().getEndLoc();
+      return SourceRange(StartL, EndL);
+    }
+
+    if (isPackExpansion())
+      return SourceRange(getFriendLoc(), getEllipsisLoc());
+
     if (NamedDecl *ND = getFriendDecl()) {
       if (const auto *FD = dyn_cast<FunctionDecl>(ND))
         return FD->getSourceRange();
@@ -158,15 +178,8 @@ class FriendDecl final
       }
       return SourceRange(getFriendLoc(), ND->getEndLoc());
     }
-    else if (TypeSourceInfo *TInfo = getFriendType()) {
-      SourceLocation StartL =
-          (NumTPLists == 0) ? getFriendLoc()
-                            : getTrailingObjects<TemplateParameterList *>()[0]
-                                  ->getTemplateLoc();
-      return SourceRange(StartL, TInfo->getTypeLoc().getEndLoc());
-    }
-    else
-      return SourceRange(getFriendLoc(), getLocation());
+
+    return SourceRange(getFriendLoc(), getLocation());
   }
 
   /// Determines if this friend kind is unsupported.
@@ -177,6 +190,8 @@ class FriendDecl final
     UnsupportedFriend = Unsupported;
   }
 
+  bool isPackExpansion() const { return EllipsisLoc.isValid(); }
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == Decl::Friend; }

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -965,6 +965,12 @@ def warn_cxx23_delete_with_message : Warning<
   "'= delete' with a message is incompatible with C++ standards before C++2c">,
   DefaultIgnore, InGroup<CXXPre26Compat>;
 
+def ext_variadic_friends : ExtWarn<
+  "variadic 'friend' declarations are a C++2c extension">, InGroup<CXX26>;
+def warn_cxx23_variadic_friends : Warning<
+  "variadic 'friend' declarations are incompatible with C++ standards before C++2c">,
+  DefaultIgnore, InGroup<CXXPre26Compat>;
+
 // C++11 default member initialization
 def ext_nonstatic_member_init : ExtWarn<
   "default member initializer for non-static data member is a C++11 "

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1741,6 +1741,10 @@ def ext_friend_tag_redecl_outside_namespace : ExtWarn<
   "enclosing namespace is a Microsoft extension; add a nested name specifier">,
   InGroup<MicrosoftUnqualifiedFriend>;
 def err_pure_friend : Error<"friend declaration cannot have a pure-specifier">;
+def err_friend_template_decl_multiple_specifiers: Error<
+  "a friend declaration that befriends a template must contain exactly one type-specifier">;
+def friend_template_decl_malformed_pack_expansion : Error<
+  "friend declaration expands pack %0 that is declared it its own template parameter list">;
 
 def err_invalid_base_in_interface : Error<
   "interface type cannot inherit from "

clang/include/clang/Sema/Sema.h

@@ -3800,7 +3800,8 @@ class Sema final : public SemaBase {
                                    const ParsedAttributesView &DeclAttrs,
                                    MultiTemplateParamsArg TemplateParams,
                                    bool IsExplicitInstantiation,
-                                   RecordDecl *&AnonRecord);
+                                   RecordDecl *&AnonRecord,
+                                   SourceLocation EllipsisLoc = {});
 
   /// BuildAnonymousStructOrUnion - Handle the declaration of an
   /// anonymous structure or union. Anonymous unions are a C++ feature
@@ -5538,7 +5539,8 @@ class Sema final : public SemaBase {
   /// parameters present at all, require proper matching, i.e.
   ///   template <> template \<class T> friend class A<int>::B;
   Decl *ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
-                            MultiTemplateParamsArg TemplateParams);
+                            MultiTemplateParamsArg TemplateParams,
+                            SourceLocation EllipsisLoc);
   NamedDecl *ActOnFriendFunctionDecl(Scope *S, Declarator &D,
                                      MultiTemplateParamsArg TemplateParams);
 
@@ -5852,6 +5854,7 @@ class Sema final : public SemaBase {
                                      unsigned TagSpec, SourceLocation TagLoc,
                                      CXXScopeSpec &SS, IdentifierInfo *Name,
                                      SourceLocation NameLoc,
+                                     SourceLocation EllipsisLoc,
                                      const ParsedAttributesView &Attr,
                                      MultiTemplateParamsArg TempParamLists);
 

clang/lib/AST/ASTImporter.cpp

@@ -4429,11 +4429,14 @@ ExpectedDecl ASTNodeImporter::VisitFriendDecl(FriendDecl *D) {
   auto FriendLocOrErr = import(D->getFriendLoc());
   if (!FriendLocOrErr)
     return FriendLocOrErr.takeError();
+  auto EllipsisLocOrErr = import(D->getEllipsisLoc());
+  if (!EllipsisLocOrErr)
+    return EllipsisLocOrErr.takeError();
 
   FriendDecl *FrD;
   if (GetImportedOrCreateDecl(FrD, D, Importer.getToContext(), DC,
-                              *LocationOrErr, ToFU,
-                              *FriendLocOrErr, ToTPLists))
+                              *LocationOrErr, ToFU, *FriendLocOrErr,
+                              *EllipsisLocOrErr, ToTPLists))
     return FrD;
 
   FrD->setAccess(D->getAccess());

clang/lib/AST/DeclFriend.cpp

@@ -31,11 +31,11 @@ FriendDecl *FriendDecl::getNextFriendSlowCase() {
                            NextFriend.get(getASTContext().getExternalSource()));
 }
 
-FriendDecl *FriendDecl::Create(ASTContext &C, DeclContext *DC,
-                               SourceLocation L,
-                               FriendUnion Friend,
-                               SourceLocation FriendL,
-                          ArrayRef<TemplateParameterList *> FriendTypeTPLists) {
+FriendDecl *
+FriendDecl::Create(ASTContext &C, DeclContext *DC, SourceLocation L,
+                   FriendUnion Friend, SourceLocation FriendL,
+                   SourceLocation EllipsisLoc,
+                   ArrayRef<TemplateParameterList *> FriendTypeTPLists) {
 #ifndef NDEBUG
   if (Friend.is<NamedDecl *>()) {
     const auto *D = Friend.get<NamedDecl*>();
@@ -56,8 +56,8 @@ FriendDecl *FriendDecl::Create(ASTContext &C, DeclContext *DC,
   std::size_t Extra =
       FriendDecl::additionalSizeToAlloc<TemplateParameterList *>(
           FriendTypeTPLists.size());
-  auto *FD = new (C, DC, Extra) FriendDecl(DC, L, Friend, FriendL,
-                                           FriendTypeTPLists);
+  auto *FD = new (C, DC, Extra)
+      FriendDecl(DC, L, Friend, FriendL, EllipsisLoc, FriendTypeTPLists);
   cast<CXXRecordDecl>(DC)->pushFriendDecl(FD);
   return FD;
 }

clang/lib/AST/DeclPrinter.cpp

@@ -868,7 +868,7 @@ void DeclPrinter::VisitFriendDecl(FriendDecl *D) {
     for (unsigned i = 0; i < NumTPLists; ++i)
       printTemplateParameters(D->getFriendTypeTemplateParameterList(i));
     Out << "friend ";
-    Out << " " << TSI->getType().getAsString(Policy);
+    Out << TSI->getType().getAsString(Policy);
   }
   else if (FunctionDecl *FD =
       dyn_cast<FunctionDecl>(D->getFriendDecl())) {
@@ -885,6 +885,9 @@ void DeclPrinter::VisitFriendDecl(FriendDecl *D) {
     Out << "friend ";
     VisitRedeclarableTemplateDecl(CTD);
   }
+
+  if (D->isPackExpansion())
+    Out << "...";
 }
 
 void DeclPrinter::VisitFieldDecl(FieldDecl *D) {

clang/lib/AST/JSONNodeDumper.cpp

@@ -1090,6 +1090,7 @@ void JSONNodeDumper::VisitAccessSpecDecl(const AccessSpecDecl *ASD) {
 void JSONNodeDumper::VisitFriendDecl(const FriendDecl *FD) {
   if (const TypeSourceInfo *T = FD->getFriendType())
     JOS.attribute("type", createQualType(T->getType()));
+  attributeOnlyIfTrue("isPackExpansion", FD->isPackExpansion());
 }
 
 void JSONNodeDumper::VisitObjCIvarDecl(const ObjCIvarDecl *D) {