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

Implement P2893R3 ‘Variadic friends’ for C++26. This closes #98587. Co-authored-by: Younan Zhang <[email protected]>
intel · Aug 15, 2024 · 2b0a8fc · 2b0a8fc
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diff --git a/clang/docs/LanguageExtensions.rst b/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

diff --git a/clang/docs/ReleaseNotes.rst b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

diff --git a/clang/include/clang/AST/DeclFriend.h b/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; }

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/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 "

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/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 "

diff --git a/clang/include/clang/Sema/Sema.h b/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);
 

diff --git a/clang/lib/AST/ASTImporter.cpp b/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());

diff --git a/clang/lib/AST/DeclFriend.cpp b/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;
 }

diff --git a/clang/lib/AST/DeclPrinter.cpp b/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) {

diff --git a/clang/lib/AST/JSONNodeDumper.cpp b/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) {

diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
@@ -461,6 +461,7 @@ class ODRDeclVisitor : public ConstDeclVisitor<ODRDeclVisitor> {
     } else {
       AddDecl(D->getFriendDecl());
     }
+    Hash.AddBoolean(D->isPackExpansion());
   }
 
   void VisitTemplateTypeParmDecl(const TemplateTypeParmDecl *D) {

diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
@@ -2697,6 +2697,8 @@ void TextNodeDumper::VisitAccessSpecDecl(const AccessSpecDecl *D) {
 void TextNodeDumper::VisitFriendDecl(const FriendDecl *D) {
   if (TypeSourceInfo *T = D->getFriendType())
     dumpType(T->getType());
+  if (D->isPackExpansion())
+    OS << "...";
 }
 
 void TextNodeDumper::VisitObjCIvarDecl(const ObjCIvarDecl *D) {

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -765,6 +765,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   // C++26 features supported in earlier language modes.
   Builder.defineMacro("__cpp_pack_indexing", "202311L");
   Builder.defineMacro("__cpp_deleted_function", "202403L");
+  Builder.defineMacro("__cpp_variadic_friend", "202403L");
 
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "202207L");

diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -452,7 +452,7 @@ Decl *Parser::ParseLinkage(ParsingDeclSpec &DS, DeclaratorContext Context) {
 ///
 ///      export-function-declaration:
 ///         'export' function-declaration
-/// 
+///
 ///      export-declaration-group:
 ///         'export' '{' function-declaration-seq[opt] '}'
 ///
@@ -2007,9 +2007,16 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
 
   const PrintingPolicy &Policy = Actions.getASTContext().getPrintingPolicy();
   TagUseKind TUK;
-  if (isDefiningTypeSpecifierContext(DSC, getLangOpts().CPlusPlus) ==
-          AllowDefiningTypeSpec::No ||
-      (getLangOpts().OpenMP && OpenMPDirectiveParsing))
+
+  // C++26 [class.mem.general]p10: If a name-declaration matches the
+  // syntactic requirements of friend-type-declaration, it is a
+  // friend-type-declaration.
+  if (getLangOpts().CPlusPlus && DS.isFriendSpecifiedFirst() &&
+      Tok.isOneOf(tok::comma, tok::ellipsis))
+    TUK = TagUseKind::Friend;
+  else if (isDefiningTypeSpecifierContext(DSC, getLangOpts().CPlusPlus) ==
+               AllowDefiningTypeSpec::No ||
+           (getLangOpts().OpenMP && OpenMPDirectiveParsing))
     TUK = TagUseKind::Reference;
   else if (Tok.is(tok::l_brace) ||
            (DSC != DeclSpecContext::DSC_association &&
@@ -2241,9 +2248,28 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
                             diag::err_keyword_not_allowed,
                             /*DiagnoseEmptyAttrs=*/true);
 
+    // Consume '...' first so we error on the ',' after it if there is one.
+    SourceLocation EllipsisLoc;
+    TryConsumeToken(tok::ellipsis, EllipsisLoc);
+
+    // CWG 2917: In a template-declaration whose declaration is a
+    // friend-type-declaration, the friend-type-specifier-list shall
+    // consist of exactly one friend-type-specifier.
+    //
+    // Essentially, the following is obviously nonsense, so disallow it:
+    //
+    //   template <typename>
+    //   friend class S, int;
+    //
+    if (Tok.is(tok::comma)) {
+      Diag(Tok.getLocation(),
+           diag::err_friend_template_decl_multiple_specifiers);
+      SkipUntil(tok::semi, StopBeforeMatch);
+    }
+
     TagOrTempResult = Actions.ActOnTemplatedFriendTag(
         getCurScope(), DS.getFriendSpecLoc(), TagType, StartLoc, SS, Name,
-        NameLoc, attrs,
+        NameLoc, EllipsisLoc, attrs,
         MultiTemplateParamsArg(TemplateParams ? &(*TemplateParams)[0] : nullptr,
                                TemplateParams ? TemplateParams->size() : 0));
   } else {
@@ -2818,6 +2844,7 @@ void Parser::MaybeParseAndDiagnoseDeclSpecAfterCXX11VirtSpecifierSeq(
 ///       member-declaration:
 ///         decl-specifier-seq[opt] member-declarator-list[opt] ';'
 ///         function-definition ';'[opt]
+/// [C++26] friend-type-declaration
 ///         ::[opt] nested-name-specifier template[opt] unqualified-id ';'[TODO]
 ///         using-declaration                                            [TODO]
 /// [C++0x] static_assert-declaration
@@ -2850,6 +2877,18 @@ void Parser::MaybeParseAndDiagnoseDeclSpecAfterCXX11VirtSpecifierSeq(
 ///       constant-initializer:
 ///         '=' constant-expression
 ///
+///       friend-type-declaration:
+///         'friend' friend-type-specifier-list ;
+///
+///       friend-type-specifier-list:
+///         friend-type-specifier ...[opt]
+///         friend-type-specifier-list , friend-type-specifier ...[opt]
+///
+///       friend-type-specifier:
+///         simple-type-specifier
+///         elaborated-type-specifier
+///         typename-specifier
+///
 Parser::DeclGroupPtrTy Parser::ParseCXXClassMemberDeclaration(
     AccessSpecifier AS, ParsedAttributes &AccessAttrs,
     ParsedTemplateInfo &TemplateInfo, ParsingDeclRAIIObject *TemplateDiags) {
@@ -3051,6 +3090,55 @@ Parser::DeclGroupPtrTy Parser::ParseCXXClassMemberDeclaration(
   if (DS.hasTagDefinition())
     Actions.ActOnDefinedDeclarationSpecifier(DS.getRepAsDecl());
 
+  // Handle C++26's variadic friend declarations. These don't even have
+  // declarators, so we get them out of the way early here.
+  if (DS.isFriendSpecifiedFirst() && Tok.isOneOf(tok::comma, tok::ellipsis)) {
+    Diag(Tok.getLocation(), getLangOpts().CPlusPlus26
+                                ? diag::warn_cxx23_variadic_friends
+                                : diag::ext_variadic_friends);
+
+    SourceLocation FriendLoc = DS.getFriendSpecLoc();
+    SmallVector<Decl *> Decls;
+
+    // Handles a single friend-type-specifier.
+    auto ParsedFriendDecl = [&](ParsingDeclSpec &DeclSpec) {
+      SourceLocation VariadicLoc;
+      TryConsumeToken(tok::ellipsis, VariadicLoc);
+
+      RecordDecl *AnonRecord = nullptr;
+      Decl *D = Actions.ParsedFreeStandingDeclSpec(
+          getCurScope(), AS, DeclSpec, DeclAttrs, TemplateParams, false,
+          AnonRecord, VariadicLoc);
+      DeclSpec.complete(D);
+      if (!D) {
+        SkipUntil(tok::semi, tok::r_brace);
+        return true;
+      }
+
+      Decls.push_back(D);
+      return false;
+    };
+
+    if (ParsedFriendDecl(DS))
+      return nullptr;
+
+    while (TryConsumeToken(tok::comma)) {
+      ParsingDeclSpec DeclSpec(*this, TemplateDiags);
+      const char *PrevSpec = nullptr;
+      unsigned DiagId = 0;
+      DeclSpec.SetFriendSpec(FriendLoc, PrevSpec, DiagId);
+      ParseDeclarationSpecifiers(DeclSpec, TemplateInfo, AS,
+                                 DeclSpecContext::DSC_class, nullptr);
+      if (ParsedFriendDecl(DeclSpec))
+        return nullptr;
+    }
+
+    ExpectAndConsume(tok::semi, diag::err_expected_semi_after_stmt,
+                     "friend declaration");
+
+    return Actions.BuildDeclaratorGroup(Decls);
+  }
+
   ParsingDeclarator DeclaratorInfo(*this, DS, DeclAttrs,
                                    DeclaratorContext::Member);
   if (TemplateInfo.TemplateParams)