clang/lib/Sema/SemaTemplateVariadic.cpp

   1 //===------- SemaTemplateVariadic.cpp - C++ Variadic Templates ------------===/
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //===----------------------------------------------------------------------===/
   7 //
   8 //  This file implements semantic analysis for C++0x variadic templates.
   9 //===----------------------------------------------------------------------===/
  10
  11 #include "clang/Sema/Sema.h"
  12 #include "TypeLocBuilder.h"
  13 #include "clang/AST/Expr.h"
  14 #include "clang/AST/RecursiveASTVisitor.h"
  15 #include "clang/AST/TypeLoc.h"
  16 #include "clang/Sema/Lookup.h"
  17 #include "clang/Sema/ParsedTemplate.h"
  18 #include "clang/Sema/ScopeInfo.h"
  19 #include "clang/Sema/SemaInternal.h"
  20 #include "clang/Sema/Template.h"
  21 #include <optional>
  22
  23 using namespace clang;
  24
  25 //----------------------------------------------------------------------------
  26 // Visitor that collects unexpanded parameter packs
  27 //----------------------------------------------------------------------------
  28
  29 namespace {
  30   /// A class that collects unexpanded parameter packs.
  31   class CollectUnexpandedParameterPacksVisitor :
  32     public RecursiveASTVisitor<CollectUnexpandedParameterPacksVisitor>
  33   {
  34     typedef RecursiveASTVisitor<CollectUnexpandedParameterPacksVisitor>
  35       inherited;
  36
  37     SmallVectorImpl<UnexpandedParameterPack> &Unexpanded;
  38
  39     bool InLambda = false;
  40     unsigned DepthLimit = (unsigned)-1;
  41
  42     void addUnexpanded(NamedDecl *ND, SourceLocation Loc = SourceLocation()) {
  43       if (auto *VD = dyn_cast<VarDecl>(ND)) {
  44         // For now, the only problematic case is a generic lambda's templated
  45         // call operator, so we don't need to look for all the other ways we
  46         // could have reached a dependent parameter pack.
  47         auto *FD = dyn_cast<FunctionDecl>(VD->getDeclContext());
  48         auto *FTD = FD ? FD->getDescribedFunctionTemplate() : nullptr;
  49         if (FTD && FTD->getTemplateParameters()->getDepth() >= DepthLimit)
  50           return;
  51       } else if (getDepthAndIndex(ND).first >= DepthLimit)
  52         return;
  53
  54       Unexpanded.push_back({ND, Loc});
  55     }
  56     void addUnexpanded(const TemplateTypeParmType *T,
  57                        SourceLocation Loc = SourceLocation()) {
  58       if (T->getDepth() < DepthLimit)
  59         Unexpanded.push_back({T, Loc});
  60     }
  61
  62   public:
  63     explicit CollectUnexpandedParameterPacksVisitor(
  64         SmallVectorImpl<UnexpandedParameterPack> &Unexpanded)
  65         : Unexpanded(Unexpanded) {}
  66
  67     bool shouldWalkTypesOfTypeLocs() const { return false; }
  68
  69     //------------------------------------------------------------------------
  70     // Recording occurrences of (unexpanded) parameter packs.
  71     //------------------------------------------------------------------------
  72
  73     /// Record occurrences of template type parameter packs.
  74     bool VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) {
  75       if (TL.getTypePtr()->isParameterPack())
  76         addUnexpanded(TL.getTypePtr(), TL.getNameLoc());
  77       return true;
  78     }
  79
  80     /// Record occurrences of template type parameter packs
  81     /// when we don't have proper source-location information for
  82     /// them.
  83     ///
  84     /// Ideally, this routine would never be used.
  85     bool VisitTemplateTypeParmType(TemplateTypeParmType *T) {
  86       if (T->isParameterPack())
  87         addUnexpanded(T);
  88
  89       return true;
  90     }
  91
  92     /// Record occurrences of function and non-type template
  93     /// parameter packs in an expression.
  94     bool VisitDeclRefExpr(DeclRefExpr *E) {
  95       if (E->getDecl()->isParameterPack())
  96         addUnexpanded(E->getDecl(), E->getLocation());
  97
  98       return true;
  99     }
 100
 101     /// Record occurrences of template template parameter packs.
 102     bool TraverseTemplateName(TemplateName Template) {
 103       if (auto *TTP = dyn_cast_or_null<TemplateTemplateParmDecl>(
 104               Template.getAsTemplateDecl())) {
 105         if (TTP->isParameterPack())
 106           addUnexpanded(TTP);
 107       }
 108
 109       return inherited::TraverseTemplateName(Template);
 110     }
 111
 112     /// Suppress traversal into Objective-C container literal
 113     /// elements that are pack expansions.
 114     bool TraverseObjCDictionaryLiteral(ObjCDictionaryLiteral *E) {
 115       if (!E->containsUnexpandedParameterPack())
 116         return true;
 117
 118       for (unsigned I = 0, N = E->getNumElements(); I != N; ++I) {
 119         ObjCDictionaryElement Element = E->getKeyValueElement(I);
 120         if (Element.isPackExpansion())
 121           continue;
 122
 123         TraverseStmt(Element.Key);
 124         TraverseStmt(Element.Value);
 125       }
 126       return true;
 127     }
 128     //------------------------------------------------------------------------
 129     // Pruning the search for unexpanded parameter packs.
 130     //------------------------------------------------------------------------
 131
 132     /// Suppress traversal into statements and expressions that
 133     /// do not contain unexpanded parameter packs.
 134     bool TraverseStmt(Stmt *S) {
 135       Expr *E = dyn_cast_or_null<Expr>(S);
 136       if ((E && E->containsUnexpandedParameterPack()) || InLambda)
 137         return inherited::TraverseStmt(S);
 138
 139       return true;
 140     }
 141
 142     /// Suppress traversal into types that do not contain
 143     /// unexpanded parameter packs.
 144     bool TraverseType(QualType T) {
 145       if ((!T.isNull() && T->containsUnexpandedParameterPack()) || InLambda)
 146         return inherited::TraverseType(T);
 147
 148       return true;
 149     }
 150
 151     /// Suppress traversal into types with location information
 152     /// that do not contain unexpanded parameter packs.
 153     bool TraverseTypeLoc(TypeLoc TL) {
 154       if ((!TL.getType().isNull() &&
 155            TL.getType()->containsUnexpandedParameterPack()) ||
 156           InLambda)
 157         return inherited::TraverseTypeLoc(TL);
 158
 159       return true;
 160     }
 161
 162     /// Suppress traversal of parameter packs.
 163     bool TraverseDecl(Decl *D) {
 164       // A function parameter pack is a pack expansion, so cannot contain
 165       // an unexpanded parameter pack. Likewise for a template parameter
 166       // pack that contains any references to other packs.
 167       if (D && D->isParameterPack())
 168         return true;
 169
 170       return inherited::TraverseDecl(D);
 171     }
 172
 173     /// Suppress traversal of pack-expanded attributes.
 174     bool TraverseAttr(Attr *A) {
 175       if (A->isPackExpansion())
 176         return true;
 177
 178       return inherited::TraverseAttr(A);
 179     }
 180
 181     /// Suppress traversal of pack expansion expressions and types.
 182     ///@{
 183     bool TraversePackExpansionType(PackExpansionType *T) { return true; }
 184     bool TraversePackExpansionTypeLoc(PackExpansionTypeLoc TL) { return true; }
 185     bool TraversePackExpansionExpr(PackExpansionExpr *E) { return true; }
 186     bool TraverseCXXFoldExpr(CXXFoldExpr *E) { return true; }
 187
 188     ///@}
 189
 190     /// Suppress traversal of using-declaration pack expansion.
 191     bool TraverseUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D) {
 192       if (D->isPackExpansion())
 193         return true;
 194
 195       return inherited::TraverseUnresolvedUsingValueDecl(D);
 196     }
 197
 198     /// Suppress traversal of using-declaration pack expansion.
 199     bool TraverseUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D) {
 200       if (D->isPackExpansion())
 201         return true;
 202
 203       return inherited::TraverseUnresolvedUsingTypenameDecl(D);
 204     }
 205
 206     /// Suppress traversal of template argument pack expansions.
 207     bool TraverseTemplateArgument(const TemplateArgument &Arg) {
 208       if (Arg.isPackExpansion())
 209         return true;
 210
 211       return inherited::TraverseTemplateArgument(Arg);
 212     }
 213
 214     /// Suppress traversal of template argument pack expansions.
 215     bool TraverseTemplateArgumentLoc(const TemplateArgumentLoc &ArgLoc) {
 216       if (ArgLoc.getArgument().isPackExpansion())
 217         return true;
 218
 219       return inherited::TraverseTemplateArgumentLoc(ArgLoc);
 220     }
 221
 222     /// Suppress traversal of base specifier pack expansions.
 223     bool TraverseCXXBaseSpecifier(const CXXBaseSpecifier &Base) {
 224       if (Base.isPackExpansion())
 225         return true;
 226
 227       return inherited::TraverseCXXBaseSpecifier(Base);
 228     }
 229
 230     /// Suppress traversal of mem-initializer pack expansions.
 231     bool TraverseConstructorInitializer(CXXCtorInitializer *Init) {
 232       if (Init->isPackExpansion())
 233         return true;
 234
 235       return inherited::TraverseConstructorInitializer(Init);
 236     }
 237
 238     /// Note whether we're traversing a lambda containing an unexpanded
 239     /// parameter pack. In this case, the unexpanded pack can occur anywhere,
 240     /// including all the places where we normally wouldn't look. Within a
 241     /// lambda, we don't propagate the 'contains unexpanded parameter pack' bit
 242     /// outside an expression.
 243     bool TraverseLambdaExpr(LambdaExpr *Lambda) {
 244       // The ContainsUnexpandedParameterPack bit on a lambda is always correct,
 245       // even if it's contained within another lambda.
 246       if (!Lambda->containsUnexpandedParameterPack())
 247         return true;
 248
 249       bool WasInLambda = InLambda;
 250       unsigned OldDepthLimit = DepthLimit;
 251
 252       InLambda = true;
 253       if (auto *TPL = Lambda->getTemplateParameterList())
 254         DepthLimit = TPL->getDepth();
 255
 256       inherited::TraverseLambdaExpr(Lambda);
 257
 258       InLambda = WasInLambda;
 259       DepthLimit = OldDepthLimit;
 260       return true;
 261     }
 262
 263     /// Suppress traversal within pack expansions in lambda captures.
 264     bool TraverseLambdaCapture(LambdaExpr *Lambda, const LambdaCapture *C,
 265                                Expr *Init) {
 266       if (C->isPackExpansion())
 267         return true;
 268
 269       return inherited::TraverseLambdaCapture(Lambda, C, Init);
 270     }
 271   };
 272 }
 273
 274 /// Determine whether it's possible for an unexpanded parameter pack to
 275 /// be valid in this location. This only happens when we're in a declaration
 276 /// that is nested within an expression that could be expanded, such as a
 277 /// lambda-expression within a function call.
 278 ///
 279 /// This is conservatively correct, but may claim that some unexpanded packs are
 280 /// permitted when they are not.
 281 bool Sema::isUnexpandedParameterPackPermitted() {
 282   for (auto *SI : FunctionScopes)
 283     if (isa<sema::LambdaScopeInfo>(SI))
 284       return true;
 285   return false;
 286 }
 287
 288 /// Diagnose all of the unexpanded parameter packs in the given
 289 /// vector.
 290 bool
 291 Sema::DiagnoseUnexpandedParameterPacks(SourceLocation Loc,
 292                                        UnexpandedParameterPackContext UPPC,
 293                                  ArrayRef<UnexpandedParameterPack> Unexpanded) {
 294   if (Unexpanded.empty())
 295     return false;
 296
 297   // If we are within a lambda expression and referencing a pack that is not
 298   // declared within the lambda itself, that lambda contains an unexpanded
 299   // parameter pack, and we are done.
 300   // FIXME: Store 'Unexpanded' on the lambda so we don't need to recompute it
 301   // later.
 302   SmallVector<UnexpandedParameterPack, 4> LambdaParamPackReferences;
 303   if (auto *LSI = getEnclosingLambda()) {
 304     for (auto &Pack : Unexpanded) {
 305       auto DeclaresThisPack = [&](NamedDecl *LocalPack) {
 306         if (auto *TTPT = Pack.first.dyn_cast<const TemplateTypeParmType *>()) {
 307           auto *TTPD = dyn_cast<TemplateTypeParmDecl>(LocalPack);
 308           return TTPD && TTPD->getTypeForDecl() == TTPT;
 309         }
 310         return declaresSameEntity(Pack.first.get<NamedDecl *>(), LocalPack);
 311       };
 312       if (llvm::any_of(LSI->LocalPacks, DeclaresThisPack))
 313         LambdaParamPackReferences.push_back(Pack);
 314     }
 315
 316     if (LambdaParamPackReferences.empty()) {
 317       // Construct in lambda only references packs declared outside the lambda.
 318       // That's OK for now, but the lambda itself is considered to contain an
 319       // unexpanded pack in this case, which will require expansion outside the
 320       // lambda.
 321
 322       // We do not permit pack expansion that would duplicate a statement
 323       // expression, not even within a lambda.
 324       // FIXME: We could probably support this for statement expressions that
 325       // do not contain labels.
 326       // FIXME: This is insufficient to detect this problem; consider
 327       //   f( ({ bad: 0; }) + pack ... );
 328       bool EnclosingStmtExpr = false;
 329       for (unsigned N = FunctionScopes.size(); N; --N) {
 330         sema::FunctionScopeInfo *Func = FunctionScopes[N-1];
 331         if (llvm::any_of(
 332                 Func->CompoundScopes,
 333                 [](sema::CompoundScopeInfo &CSI) { return CSI.IsStmtExpr; })) {
 334           EnclosingStmtExpr = true;
 335           break;
 336         }
 337         // Coumpound-statements outside the lambda are OK for now; we'll check
 338         // for those when we finish handling the lambda.
 339         if (Func == LSI)
 340           break;
 341       }
 342
 343       if (!EnclosingStmtExpr) {
 344         LSI->ContainsUnexpandedParameterPack = true;
 345         return false;
 346       }
 347     } else {
 348       Unexpanded = LambdaParamPackReferences;
 349     }
 350   }
 351
 352   SmallVector<SourceLocation, 4> Locations;
 353   SmallVector<IdentifierInfo *, 4> Names;
 354   llvm::SmallPtrSet<IdentifierInfo *, 4> NamesKnown;
 355
 356   for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
 357     IdentifierInfo *Name = nullptr;
 358     if (const TemplateTypeParmType *TTP
 359           = Unexpanded[I].first.dyn_cast<const TemplateTypeParmType *>())
 360       Name = TTP->getIdentifier();
 361     else
 362       Name = Unexpanded[I].first.get<NamedDecl *>()->getIdentifier();
 363
 364     if (Name && NamesKnown.insert(Name).second)
 365       Names.push_back(Name);
 366
 367     if (Unexpanded[I].second.isValid())
 368       Locations.push_back(Unexpanded[I].second);
 369   }
 370
 371   auto DB = Diag(Loc, diag::err_unexpanded_parameter_pack)
 372             << (int)UPPC << (int)Names.size();
 373   for (size_t I = 0, E = std::min(Names.size(), (size_t)2); I != E; ++I)
 374     DB << Names[I];
 375
 376   for (unsigned I = 0, N = Locations.size(); I != N; ++I)
 377     DB << SourceRange(Locations[I]);
 378   return true;
 379 }
 380
 381 bool Sema::DiagnoseUnexpandedParameterPack(SourceLocation Loc,
 382                                            TypeSourceInfo *T,
 383                                          UnexpandedParameterPackContext UPPC) {
 384   // C++0x [temp.variadic]p5:
 385   //   An appearance of a name of a parameter pack that is not expanded is
 386   //   ill-formed.
 387   if (!T->getType()->containsUnexpandedParameterPack())
 388     return false;
 389
 390   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 391   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseTypeLoc(
 392                                                               T->getTypeLoc());
 393   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 394   return DiagnoseUnexpandedParameterPacks(Loc, UPPC, Unexpanded);
 395 }
 396
 397 bool Sema::DiagnoseUnexpandedParameterPack(Expr *E,
 398                                         UnexpandedParameterPackContext UPPC) {
 399   // C++0x [temp.variadic]p5:
 400   //   An appearance of a name of a parameter pack that is not expanded is
 401   //   ill-formed.
 402   if (!E->containsUnexpandedParameterPack())
 403     return false;
 404
 405   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 406   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseStmt(E);
 407   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 408   return DiagnoseUnexpandedParameterPacks(E->getBeginLoc(), UPPC, Unexpanded);
 409 }
 410
 411 bool Sema::DiagnoseUnexpandedParameterPackInRequiresExpr(RequiresExpr *RE) {
 412   if (!RE->containsUnexpandedParameterPack())
 413     return false;
 414
 415   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 416   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseStmt(RE);
 417   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 418
 419   // We only care about unexpanded references to the RequiresExpr's own
 420   // parameter packs.
 421   auto Parms = RE->getLocalParameters();
 422   llvm::SmallPtrSet<NamedDecl*, 8> ParmSet(Parms.begin(), Parms.end());
 423   SmallVector<UnexpandedParameterPack, 2> UnexpandedParms;
 424   for (auto Parm : Unexpanded)
 425     if (ParmSet.contains(Parm.first.dyn_cast<NamedDecl *>()))
 426       UnexpandedParms.push_back(Parm);
 427   if (UnexpandedParms.empty())
 428     return false;
 429
 430   return DiagnoseUnexpandedParameterPacks(RE->getBeginLoc(), UPPC_Requirement,
 431                                           UnexpandedParms);
 432 }
 433
 434 bool Sema::DiagnoseUnexpandedParameterPack(const CXXScopeSpec &SS,
 435                                         UnexpandedParameterPackContext UPPC) {
 436   // C++0x [temp.variadic]p5:
 437   //   An appearance of a name of a parameter pack that is not expanded is
 438   //   ill-formed.
 439   if (!SS.getScopeRep() ||
 440       !SS.getScopeRep()->containsUnexpandedParameterPack())
 441     return false;
 442
 443   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 444   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 445     .TraverseNestedNameSpecifier(SS.getScopeRep());
 446   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 447   return DiagnoseUnexpandedParameterPacks(SS.getRange().getBegin(),
 448                                           UPPC, Unexpanded);
 449 }
 450
 451 bool Sema::DiagnoseUnexpandedParameterPack(const DeclarationNameInfo &NameInfo,
 452                                          UnexpandedParameterPackContext UPPC) {
 453   // C++0x [temp.variadic]p5:
 454   //   An appearance of a name of a parameter pack that is not expanded is
 455   //   ill-formed.
 456   switch (NameInfo.getName().getNameKind()) {
 457   case DeclarationName::Identifier:
 458   case DeclarationName::ObjCZeroArgSelector:
 459   case DeclarationName::ObjCOneArgSelector:
 460   case DeclarationName::ObjCMultiArgSelector:
 461   case DeclarationName::CXXOperatorName:
 462   case DeclarationName::CXXLiteralOperatorName:
 463   case DeclarationName::CXXUsingDirective:
 464   case DeclarationName::CXXDeductionGuideName:
 465     return false;
 466
 467   case DeclarationName::CXXConstructorName:
 468   case DeclarationName::CXXDestructorName:
 469   case DeclarationName::CXXConversionFunctionName:
 470     // FIXME: We shouldn't need this null check!
 471     if (TypeSourceInfo *TSInfo = NameInfo.getNamedTypeInfo())
 472       return DiagnoseUnexpandedParameterPack(NameInfo.getLoc(), TSInfo, UPPC);
 473
 474     if (!NameInfo.getName().getCXXNameType()->containsUnexpandedParameterPack())
 475       return false;
 476
 477     break;
 478   }
 479
 480   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 481   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 482     .TraverseType(NameInfo.getName().getCXXNameType());
 483   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 484   return DiagnoseUnexpandedParameterPacks(NameInfo.getLoc(), UPPC, Unexpanded);
 485 }
 486
 487 bool Sema::DiagnoseUnexpandedParameterPack(SourceLocation Loc,
 488                                            TemplateName Template,
 489                                        UnexpandedParameterPackContext UPPC) {
 490
 491   if (Template.isNull() || !Template.containsUnexpandedParameterPack())
 492     return false;
 493
 494   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 495   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 496     .TraverseTemplateName(Template);
 497   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 498   return DiagnoseUnexpandedParameterPacks(Loc, UPPC, Unexpanded);
 499 }
 500
 501 bool Sema::DiagnoseUnexpandedParameterPack(TemplateArgumentLoc Arg,
 502                                          UnexpandedParameterPackContext UPPC) {
 503   if (Arg.getArgument().isNull() ||
 504       !Arg.getArgument().containsUnexpandedParameterPack())
 505     return false;
 506
 507   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 508   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 509     .TraverseTemplateArgumentLoc(Arg);
 510   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
 511   return DiagnoseUnexpandedParameterPacks(Arg.getLocation(), UPPC, Unexpanded);
 512 }
 513
 514 void Sema::collectUnexpandedParameterPacks(TemplateArgument Arg,
 515                    SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 516   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 517     .TraverseTemplateArgument(Arg);
 518 }
 519
 520 void Sema::collectUnexpandedParameterPacks(TemplateArgumentLoc Arg,
 521                    SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 522   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 523     .TraverseTemplateArgumentLoc(Arg);
 524 }
 525
 526 void Sema::collectUnexpandedParameterPacks(QualType T,
 527                    SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 528   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseType(T);
 529 }
 530
 531 void Sema::collectUnexpandedParameterPacks(TypeLoc TL,
 532                    SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 533   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseTypeLoc(TL);
 534 }
 535
 536 void Sema::collectUnexpandedParameterPacks(
 537     NestedNameSpecifierLoc NNS,
 538     SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 539   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 540       .TraverseNestedNameSpecifierLoc(NNS);
 541 }
 542
 543 void Sema::collectUnexpandedParameterPacks(
 544     const DeclarationNameInfo &NameInfo,
 545     SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
 546   CollectUnexpandedParameterPacksVisitor(Unexpanded)
 547     .TraverseDeclarationNameInfo(NameInfo);
 548 }
 549
 550
 551 ParsedTemplateArgument
 552 Sema::ActOnPackExpansion(const ParsedTemplateArgument &Arg,
 553                          SourceLocation EllipsisLoc) {
 554   if (Arg.isInvalid())
 555     return Arg;
 556
 557   switch (Arg.getKind()) {
 558   case ParsedTemplateArgument::Type: {
 559     TypeResult Result = ActOnPackExpansion(Arg.getAsType(), EllipsisLoc);
 560     if (Result.isInvalid())
 561       return ParsedTemplateArgument();
 562
 563     return ParsedTemplateArgument(Arg.getKind(), Result.get().getAsOpaquePtr(),
 564                                   Arg.getLocation());
 565   }
 566
 567   case ParsedTemplateArgument::NonType: {
 568     ExprResult Result = ActOnPackExpansion(Arg.getAsExpr(), EllipsisLoc);
 569     if (Result.isInvalid())
 570       return ParsedTemplateArgument();
 571
 572     return ParsedTemplateArgument(Arg.getKind(), Result.get(),
 573                                   Arg.getLocation());
 574   }
 575
 576   case ParsedTemplateArgument::Template:
 577     if (!Arg.getAsTemplate().get().containsUnexpandedParameterPack()) {
 578       SourceRange R(Arg.getLocation());
 579       if (Arg.getScopeSpec().isValid())
 580         R.setBegin(Arg.getScopeSpec().getBeginLoc());
 581       Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
 582         << R;
 583       return ParsedTemplateArgument();
 584     }
 585
 586     return Arg.getTemplatePackExpansion(EllipsisLoc);
 587   }
 588   llvm_unreachable("Unhandled template argument kind?");
 589 }
 590
 591 TypeResult Sema::ActOnPackExpansion(ParsedType Type,
 592                                     SourceLocation EllipsisLoc) {
 593   TypeSourceInfo *TSInfo;
 594   GetTypeFromParser(Type, &TSInfo);
 595   if (!TSInfo)
 596     return true;
 597
 598   TypeSourceInfo *TSResult =
 599       CheckPackExpansion(TSInfo, EllipsisLoc, std::nullopt);
 600   if (!TSResult)
 601     return true;
 602
 603   return CreateParsedType(TSResult->getType(), TSResult);
 604 }
 605
 606 TypeSourceInfo *
 607 Sema::CheckPackExpansion(TypeSourceInfo *Pattern, SourceLocation EllipsisLoc,
 608                          std::optional<unsigned> NumExpansions) {
 609   // Create the pack expansion type and source-location information.
 610   QualType Result = CheckPackExpansion(Pattern->getType(),
 611                                        Pattern->getTypeLoc().getSourceRange(),
 612                                        EllipsisLoc, NumExpansions);
 613   if (Result.isNull())
 614     return nullptr;
 615
 616   TypeLocBuilder TLB;
 617   TLB.pushFullCopy(Pattern->getTypeLoc());
 618   PackExpansionTypeLoc TL = TLB.push<PackExpansionTypeLoc>(Result);
 619   TL.setEllipsisLoc(EllipsisLoc);
 620
 621   return TLB.getTypeSourceInfo(Context, Result);
 622 }
 623
 624 QualType Sema::CheckPackExpansion(QualType Pattern, SourceRange PatternRange,
 625                                   SourceLocation EllipsisLoc,
 626                                   std::optional<unsigned> NumExpansions) {
 627   // C++11 [temp.variadic]p5:
 628   //   The pattern of a pack expansion shall name one or more
 629   //   parameter packs that are not expanded by a nested pack
 630   //   expansion.
 631   //
 632   // A pattern containing a deduced type can't occur "naturally" but arises in
 633   // the desugaring of an init-capture pack.
 634   if (!Pattern->containsUnexpandedParameterPack() &&
 635       !Pattern->getContainedDeducedType()) {
 636     Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
 637       << PatternRange;
 638     return QualType();
 639   }
 640
 641   return Context.getPackExpansionType(Pattern, NumExpansions,
 642                                       /*ExpectPackInType=*/false);
 643 }
 644
 645 ExprResult Sema::ActOnPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc) {
 646   return CheckPackExpansion(Pattern, EllipsisLoc, std::nullopt);
 647 }
 648
 649 ExprResult Sema::CheckPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc,
 650                                     std::optional<unsigned> NumExpansions) {
 651   if (!Pattern)
 652     return ExprError();
 653
 654   // C++0x [temp.variadic]p5:
 655   //   The pattern of a pack expansion shall name one or more
 656   //   parameter packs that are not expanded by a nested pack
 657   //   expansion.
 658   if (!Pattern->containsUnexpandedParameterPack()) {
 659     Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
 660     << Pattern->getSourceRange();
 661     CorrectDelayedTyposInExpr(Pattern);
 662     return ExprError();
 663   }
 664
 665   // Create the pack expansion expression and source-location information.
 666   return new (Context)
 667     PackExpansionExpr(Context.DependentTy, Pattern, EllipsisLoc, NumExpansions);
 668 }
 669
 670 bool Sema::CheckParameterPacksForExpansion(
 671     SourceLocation EllipsisLoc, SourceRange PatternRange,
 672     ArrayRef<UnexpandedParameterPack> Unexpanded,
 673     const MultiLevelTemplateArgumentList &TemplateArgs, bool &ShouldExpand,
 674     bool &RetainExpansion, std::optional<unsigned> &NumExpansions) {
 675   ShouldExpand = true;
 676   RetainExpansion = false;
 677   std::pair<IdentifierInfo *, SourceLocation> FirstPack;
 678   bool HaveFirstPack = false;
 679   std::optional<unsigned> NumPartialExpansions;
 680   SourceLocation PartiallySubstitutedPackLoc;
 681
 682   for (UnexpandedParameterPack ParmPack : Unexpanded) {
 683     // Compute the depth and index for this parameter pack.
 684     unsigned Depth = 0, Index = 0;
 685     IdentifierInfo *Name;
 686     bool IsVarDeclPack = false;
 687
 688     if (const TemplateTypeParmType *TTP =
 689             ParmPack.first.dyn_cast<const TemplateTypeParmType *>()) {
 690       Depth = TTP->getDepth();
 691       Index = TTP->getIndex();
 692       Name = TTP->getIdentifier();
 693     } else {
 694       NamedDecl *ND = ParmPack.first.get<NamedDecl *>();
 695       if (isa<VarDecl>(ND))
 696         IsVarDeclPack = true;
 697       else
 698         std::tie(Depth, Index) = getDepthAndIndex(ND);
 699
 700       Name = ND->getIdentifier();
 701     }
 702
 703     // Determine the size of this argument pack.
 704     unsigned NewPackSize;
 705     if (IsVarDeclPack) {
 706       // Figure out whether we're instantiating to an argument pack or not.
 707       typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
 708
 709       llvm::PointerUnion<Decl *, DeclArgumentPack *> *Instantiation =
 710           CurrentInstantiationScope->findInstantiationOf(
 711               ParmPack.first.get<NamedDecl *>());
 712       if (Instantiation->is<DeclArgumentPack *>()) {
 713         // We could expand this function parameter pack.
 714         NewPackSize = Instantiation->get<DeclArgumentPack *>()->size();
 715       } else {
 716         // We can't expand this function parameter pack, so we can't expand
 717         // the pack expansion.
 718         ShouldExpand = false;
 719         continue;
 720       }
 721     } else {
 722       // If we don't have a template argument at this depth/index, then we
 723       // cannot expand the pack expansion. Make a note of this, but we still
 724       // want to check any parameter packs we *do* have arguments for.
 725       if (Depth >= TemplateArgs.getNumLevels() ||
 726           !TemplateArgs.hasTemplateArgument(Depth, Index)) {
 727         ShouldExpand = false;
 728         continue;
 729       }
 730
 731       // Determine the size of the argument pack.
 732       NewPackSize = TemplateArgs(Depth, Index).pack_size();
 733     }
 734
 735     // C++0x [temp.arg.explicit]p9:
 736     //   Template argument deduction can extend the sequence of template
 737     //   arguments corresponding to a template parameter pack, even when the
 738     //   sequence contains explicitly specified template arguments.
 739     if (!IsVarDeclPack && CurrentInstantiationScope) {
 740       if (NamedDecl *PartialPack =
 741               CurrentInstantiationScope->getPartiallySubstitutedPack()) {
 742         unsigned PartialDepth, PartialIndex;
 743         std::tie(PartialDepth, PartialIndex) = getDepthAndIndex(PartialPack);
 744         if (PartialDepth == Depth && PartialIndex == Index) {
 745           RetainExpansion = true;
 746           // We don't actually know the new pack size yet.
 747           NumPartialExpansions = NewPackSize;
 748           PartiallySubstitutedPackLoc = ParmPack.second;
 749           continue;
 750         }
 751       }
 752     }
 753
 754     if (!NumExpansions) {
 755       // The is the first pack we've seen for which we have an argument.
 756       // Record it.
 757       NumExpansions = NewPackSize;
 758       FirstPack.first = Name;
 759       FirstPack.second = ParmPack.second;
 760       HaveFirstPack = true;
 761       continue;
 762     }
 763
 764     if (NewPackSize != *NumExpansions) {
 765       // C++0x [temp.variadic]p5:
 766       //   All of the parameter packs expanded by a pack expansion shall have
 767       //   the same number of arguments specified.
 768       if (HaveFirstPack)
 769         Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict)
 770             << FirstPack.first << Name << *NumExpansions << NewPackSize
 771             << SourceRange(FirstPack.second) << SourceRange(ParmPack.second);
 772       else
 773         Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict_multilevel)
 774             << Name << *NumExpansions << NewPackSize
 775             << SourceRange(ParmPack.second);
 776       return true;
 777     }
 778   }
 779
 780   // If we're performing a partial expansion but we also have a full expansion,
 781   // expand to the number of common arguments. For example, given:
 782   //
 783   //   template<typename ...T> struct A {
 784   //     template<typename ...U> void f(pair<T, U>...);
 785   //   };
 786   //
 787   // ... a call to 'A<int, int>().f<int>' should expand the pack once and
 788   // retain an expansion.
 789   if (NumPartialExpansions) {
 790     if (NumExpansions && *NumExpansions < *NumPartialExpansions) {
 791       NamedDecl *PartialPack =
 792           CurrentInstantiationScope->getPartiallySubstitutedPack();
 793       Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict_partial)
 794           << PartialPack << *NumPartialExpansions << *NumExpansions
 795           << SourceRange(PartiallySubstitutedPackLoc);
 796       return true;
 797     }
 798
 799     NumExpansions = NumPartialExpansions;
 800   }
 801
 802   return false;
 803 }
 804
 805 std::optional<unsigned> Sema::getNumArgumentsInExpansion(
 806     QualType T, const MultiLevelTemplateArgumentList &TemplateArgs) {
 807   QualType Pattern = cast<PackExpansionType>(T)->getPattern();
 808   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
 809   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseType(Pattern);
 810
 811   std::optional<unsigned> Result;
 812   for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
 813     // Compute the depth and index for this parameter pack.
 814     unsigned Depth;
 815     unsigned Index;
 816
 817     if (const TemplateTypeParmType *TTP =
 818             Unexpanded[I].first.dyn_cast<const TemplateTypeParmType *>()) {
 819       Depth = TTP->getDepth();
 820       Index = TTP->getIndex();
 821     } else {
 822       NamedDecl *ND = Unexpanded[I].first.get<NamedDecl *>();
 823       if (isa<VarDecl>(ND)) {
 824         // Function parameter pack or init-capture pack.
 825         typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
 826
 827         llvm::PointerUnion<Decl *, DeclArgumentPack *> *Instantiation =
 828             CurrentInstantiationScope->findInstantiationOf(
 829                 Unexpanded[I].first.get<NamedDecl *>());
 830         if (Instantiation->is<Decl *>())
 831           // The pattern refers to an unexpanded pack. We're not ready to expand
 832           // this pack yet.
 833           return std::nullopt;
 834
 835         unsigned Size = Instantiation->get<DeclArgumentPack *>()->size();
 836         assert((!Result || *Result == Size) && "inconsistent pack sizes");
 837         Result = Size;
 838         continue;
 839       }
 840
 841       std::tie(Depth, Index) = getDepthAndIndex(ND);
 842     }
 843     if (Depth >= TemplateArgs.getNumLevels() ||
 844         !TemplateArgs.hasTemplateArgument(Depth, Index))
 845       // The pattern refers to an unknown template argument. We're not ready to
 846       // expand this pack yet.
 847       return std::nullopt;
 848
 849     // Determine the size of the argument pack.
 850     unsigned Size = TemplateArgs(Depth, Index).pack_size();
 851     assert((!Result || *Result == Size) && "inconsistent pack sizes");
 852     Result = Size;
 853   }
 854
 855   return Result;
 856 }
 857
 858 bool Sema::containsUnexpandedParameterPacks(Declarator &D) {
 859   const DeclSpec &DS = D.getDeclSpec();
 860   switch (DS.getTypeSpecType()) {
 861   case TST_typename:
 862   case TST_typeof_unqualType:
 863   case TST_typeofType:
 864 #define TRANSFORM_TYPE_TRAIT_DEF(_, Trait) case TST_##Trait:
 865 #include "clang/Basic/TransformTypeTraits.def"
 866   case TST_atomic: {
 867     QualType T = DS.getRepAsType().get();
 868     if (!T.isNull() && T->containsUnexpandedParameterPack())
 869       return true;
 870     break;
 871   }
 872
 873   case TST_typeof_unqualExpr:
 874   case TST_typeofExpr:
 875   case TST_decltype:
 876   case TST_bitint:
 877     if (DS.getRepAsExpr() &&
 878         DS.getRepAsExpr()->containsUnexpandedParameterPack())
 879       return true;
 880     break;
 881
 882   case TST_unspecified:
 883   case TST_void:
 884   case TST_char:
 885   case TST_wchar:
 886   case TST_char8:
 887   case TST_char16:
 888   case TST_char32:
 889   case TST_int:
 890   case TST_int128:
 891   case TST_half:
 892   case TST_float:
 893   case TST_double:
 894   case TST_Accum:
 895   case TST_Fract:
 896   case TST_Float16:
 897   case TST_float128:
 898   case TST_ibm128:
 899   case TST_bool:
 900   case TST_decimal32:
 901   case TST_decimal64:
 902   case TST_decimal128:
 903   case TST_enum:
 904   case TST_union:
 905   case TST_struct:
 906   case TST_interface:
 907   case TST_class:
 908   case TST_auto:
 909   case TST_auto_type:
 910   case TST_decltype_auto:
 911   case TST_BFloat16:
 912 #define GENERIC_IMAGE_TYPE(ImgType, Id) case TST_##ImgType##_t:
 913 #include "clang/Basic/OpenCLImageTypes.def"
 914   case TST_unknown_anytype:
 915   case TST_error:
 916     break;
 917   }
 918
 919   for (unsigned I = 0, N = D.getNumTypeObjects(); I != N; ++I) {
 920     const DeclaratorChunk &Chunk = D.getTypeObject(I);
 921     switch (Chunk.Kind) {
 922     case DeclaratorChunk::Pointer:
 923     case DeclaratorChunk::Reference:
 924     case DeclaratorChunk::Paren:
 925     case DeclaratorChunk::Pipe:
 926     case DeclaratorChunk::BlockPointer:
 927       // These declarator chunks cannot contain any parameter packs.
 928       break;
 929
 930     case DeclaratorChunk::Array:
 931       if (Chunk.Arr.NumElts &&
 932           Chunk.Arr.NumElts->containsUnexpandedParameterPack())
 933         return true;
 934       break;
 935     case DeclaratorChunk::Function:
 936       for (unsigned i = 0, e = Chunk.Fun.NumParams; i != e; ++i) {
 937         ParmVarDecl *Param = cast<ParmVarDecl>(Chunk.Fun.Params[i].Param);
 938         QualType ParamTy = Param->getType();
 939         assert(!ParamTy.isNull() && "Couldn't parse type?");
 940         if (ParamTy->containsUnexpandedParameterPack()) return true;
 941       }
 942
 943       if (Chunk.Fun.getExceptionSpecType() == EST_Dynamic) {
 944         for (unsigned i = 0; i != Chunk.Fun.getNumExceptions(); ++i) {
 945           if (Chunk.Fun.Exceptions[i]
 946                   .Ty.get()
 947                   ->containsUnexpandedParameterPack())
 948             return true;
 949         }
 950       } else if (isComputedNoexcept(Chunk.Fun.getExceptionSpecType()) &&
 951                  Chunk.Fun.NoexceptExpr->containsUnexpandedParameterPack())
 952         return true;
 953
 954       if (Chunk.Fun.hasTrailingReturnType()) {
 955         QualType T = Chunk.Fun.getTrailingReturnType().get();
 956         if (!T.isNull() && T->containsUnexpandedParameterPack())
 957           return true;
 958       }
 959       break;
 960
 961     case DeclaratorChunk::MemberPointer:
 962       if (Chunk.Mem.Scope().getScopeRep() &&
 963           Chunk.Mem.Scope().getScopeRep()->containsUnexpandedParameterPack())
 964         return true;
 965       break;
 966     }
 967   }
 968
 969   if (Expr *TRC = D.getTrailingRequiresClause())
 970     if (TRC->containsUnexpandedParameterPack())
 971       return true;
 972
 973   return false;
 974 }
 975
 976 namespace {
 977
 978 // Callback to only accept typo corrections that refer to parameter packs.
 979 class ParameterPackValidatorCCC final : public CorrectionCandidateCallback {
 980  public:
 981   bool ValidateCandidate(const TypoCorrection &candidate) override {
 982     NamedDecl *ND = candidate.getCorrectionDecl();
 983     return ND && ND->isParameterPack();
 984   }
 985
 986   std::unique_ptr<CorrectionCandidateCallback> clone() override {
 987     return std::make_unique<ParameterPackValidatorCCC>(*this);
 988   }
 989 };
 990
 991 }
 992
 993 /// Called when an expression computing the size of a parameter pack
 994 /// is parsed.
 995 ///
 996 /// \code
 997 /// template<typename ...Types> struct count {
 998 ///   static const unsigned value = sizeof...(Types);
 999 /// };
1000 /// \endcode
1001 ///
1002 //
1003 /// \param OpLoc The location of the "sizeof" keyword.
1004 /// \param Name The name of the parameter pack whose size will be determined.
1005 /// \param NameLoc The source location of the name of the parameter pack.
1006 /// \param RParenLoc The location of the closing parentheses.
1007 ExprResult Sema::ActOnSizeofParameterPackExpr(Scope *S,
1008                                               SourceLocation OpLoc,
1009                                               IdentifierInfo &Name,
1010                                               SourceLocation NameLoc,
1011                                               SourceLocation RParenLoc) {
1012   // C++0x [expr.sizeof]p5:
1013   //   The identifier in a sizeof... expression shall name a parameter pack.
1014   LookupResult R(*this, &Name, NameLoc, LookupOrdinaryName);
1015   LookupName(R, S);
1016
1017   NamedDecl *ParameterPack = nullptr;
1018   switch (R.getResultKind()) {
1019   case LookupResult::Found:
1020     ParameterPack = R.getFoundDecl();
1021     break;
1022
1023   case LookupResult::NotFound:
1024   case LookupResult::NotFoundInCurrentInstantiation: {
1025     ParameterPackValidatorCCC CCC{};
1026     if (TypoCorrection Corrected =
1027             CorrectTypo(R.getLookupNameInfo(), R.getLookupKind(), S, nullptr,
1028                         CCC, CTK_ErrorRecovery)) {
1029       diagnoseTypo(Corrected,
1030                    PDiag(diag::err_sizeof_pack_no_pack_name_suggest) << &Name,
1031                    PDiag(diag::note_parameter_pack_here));
1032       ParameterPack = Corrected.getCorrectionDecl();
1033     }
1034     break;
1035   }
1036   case LookupResult::FoundOverloaded:
1037   case LookupResult::FoundUnresolvedValue:
1038     break;
1039
1040   case LookupResult::Ambiguous:
1041     DiagnoseAmbiguousLookup(R);
1042     return ExprError();
1043   }
1044
1045   if (!ParameterPack || !ParameterPack->isParameterPack()) {
1046     Diag(NameLoc, diag::err_sizeof_pack_no_pack_name)
1047       << &Name;
1048     return ExprError();
1049   }
1050
1051   MarkAnyDeclReferenced(OpLoc, ParameterPack, true);
1052
1053   return SizeOfPackExpr::Create(Context, OpLoc, ParameterPack, NameLoc,
1054                                 RParenLoc);
1055 }
1056
1057 TemplateArgumentLoc Sema::getTemplateArgumentPackExpansionPattern(
1058     TemplateArgumentLoc OrigLoc, SourceLocation &Ellipsis,
1059     std::optional<unsigned> &NumExpansions) const {
1060   const TemplateArgument &Argument = OrigLoc.getArgument();
1061   assert(Argument.isPackExpansion());
1062   switch (Argument.getKind()) {
1063   case TemplateArgument::Type: {
1064     // FIXME: We shouldn't ever have to worry about missing
1065     // type-source info!
1066     TypeSourceInfo *ExpansionTSInfo = OrigLoc.getTypeSourceInfo();
1067     if (!ExpansionTSInfo)
1068       ExpansionTSInfo = Context.getTrivialTypeSourceInfo(Argument.getAsType(),
1069                                                          Ellipsis);
1070     PackExpansionTypeLoc Expansion =
1071         ExpansionTSInfo->getTypeLoc().castAs<PackExpansionTypeLoc>();
1072     Ellipsis = Expansion.getEllipsisLoc();
1073
1074     TypeLoc Pattern = Expansion.getPatternLoc();
1075     NumExpansions = Expansion.getTypePtr()->getNumExpansions();
1076
1077     // We need to copy the TypeLoc because TemplateArgumentLocs store a
1078     // TypeSourceInfo.
1079     // FIXME: Find some way to avoid the copy?
1080     TypeLocBuilder TLB;
1081     TLB.pushFullCopy(Pattern);
1082     TypeSourceInfo *PatternTSInfo =
1083         TLB.getTypeSourceInfo(Context, Pattern.getType());
1084     return TemplateArgumentLoc(TemplateArgument(Pattern.getType()),
1085                                PatternTSInfo);
1086   }
1087
1088   case TemplateArgument::Expression: {
1089     PackExpansionExpr *Expansion
1090       = cast<PackExpansionExpr>(Argument.getAsExpr());
1091     Expr *Pattern = Expansion->getPattern();
1092     Ellipsis = Expansion->getEllipsisLoc();
1093     NumExpansions = Expansion->getNumExpansions();
1094     return TemplateArgumentLoc(Pattern, Pattern);
1095   }
1096
1097   case TemplateArgument::TemplateExpansion:
1098     Ellipsis = OrigLoc.getTemplateEllipsisLoc();
1099     NumExpansions = Argument.getNumTemplateExpansions();
1100     return TemplateArgumentLoc(Context, Argument.getPackExpansionPattern(),
1101                                OrigLoc.getTemplateQualifierLoc(),
1102                                OrigLoc.getTemplateNameLoc());
1103
1104   case TemplateArgument::Declaration:
1105   case TemplateArgument::NullPtr:
1106   case TemplateArgument::Template:
1107   case TemplateArgument::Integral:
1108   case TemplateArgument::Pack:
1109   case TemplateArgument::Null:
1110     return TemplateArgumentLoc();
1111   }
1112
1113   llvm_unreachable("Invalid TemplateArgument Kind!");
1114 }
1115
1116 std::optional<unsigned> Sema::getFullyPackExpandedSize(TemplateArgument Arg) {
1117   assert(Arg.containsUnexpandedParameterPack());
1118
1119   // If this is a substituted pack, grab that pack. If not, we don't know
1120   // the size yet.
1121   // FIXME: We could find a size in more cases by looking for a substituted
1122   // pack anywhere within this argument, but that's not necessary in the common
1123   // case for 'sizeof...(A)' handling.
1124   TemplateArgument Pack;
1125   switch (Arg.getKind()) {
1126   case TemplateArgument::Type:
1127     if (auto *Subst = Arg.getAsType()->getAs<SubstTemplateTypeParmPackType>())
1128       Pack = Subst->getArgumentPack();
1129     else
1130       return std::nullopt;
1131     break;
1132
1133   case TemplateArgument::Expression:
1134     if (auto *Subst =
1135             dyn_cast<SubstNonTypeTemplateParmPackExpr>(Arg.getAsExpr()))
1136       Pack = Subst->getArgumentPack();
1137     else if (auto *Subst = dyn_cast<FunctionParmPackExpr>(Arg.getAsExpr()))  {
1138       for (VarDecl *PD : *Subst)
1139         if (PD->isParameterPack())
1140           return std::nullopt;
1141       return Subst->getNumExpansions();
1142     } else
1143       return std::nullopt;
1144     break;
1145
1146   case TemplateArgument::Template:
1147     if (SubstTemplateTemplateParmPackStorage *Subst =
1148             Arg.getAsTemplate().getAsSubstTemplateTemplateParmPack())
1149       Pack = Subst->getArgumentPack();
1150     else
1151       return std::nullopt;
1152     break;
1153
1154   case TemplateArgument::Declaration:
1155   case TemplateArgument::NullPtr:
1156   case TemplateArgument::TemplateExpansion:
1157   case TemplateArgument::Integral:
1158   case TemplateArgument::Pack:
1159   case TemplateArgument::Null:
1160     return std::nullopt;
1161   }
1162
1163   // Check that no argument in the pack is itself a pack expansion.
1164   for (TemplateArgument Elem : Pack.pack_elements()) {
1165     // There's no point recursing in this case; we would have already
1166     // expanded this pack expansion into the enclosing pack if we could.
1167     if (Elem.isPackExpansion())
1168       return std::nullopt;
1169   }
1170   return Pack.pack_size();
1171 }
1172
1173 static void CheckFoldOperand(Sema &S, Expr *E) {
1174   if (!E)
1175     return;
1176
1177   E = E->IgnoreImpCasts();
1178   auto *OCE = dyn_cast<CXXOperatorCallExpr>(E);
1179   if ((OCE && OCE->isInfixBinaryOp()) || isa<BinaryOperator>(E) ||
1180       isa<AbstractConditionalOperator>(E)) {
1181     S.Diag(E->getExprLoc(), diag::err_fold_expression_bad_operand)
1182         << E->getSourceRange()
1183         << FixItHint::CreateInsertion(E->getBeginLoc(), "(")
1184         << FixItHint::CreateInsertion(E->getEndLoc(), ")");
1185   }
1186 }
1187
1188 ExprResult Sema::ActOnCXXFoldExpr(Scope *S, SourceLocation LParenLoc, Expr *LHS,
1189                                   tok::TokenKind Operator,
1190                                   SourceLocation EllipsisLoc, Expr *RHS,
1191                                   SourceLocation RParenLoc) {
1192   // LHS and RHS must be cast-expressions. We allow an arbitrary expression
1193   // in the parser and reduce down to just cast-expressions here.
1194   CheckFoldOperand(*this, LHS);
1195   CheckFoldOperand(*this, RHS);
1196
1197   auto DiscardOperands = [&] {
1198     CorrectDelayedTyposInExpr(LHS);
1199     CorrectDelayedTyposInExpr(RHS);
1200   };
1201
1202   // [expr.prim.fold]p3:
1203   //   In a binary fold, op1 and op2 shall be the same fold-operator, and
1204   //   either e1 shall contain an unexpanded parameter pack or e2 shall contain
1205   //   an unexpanded parameter pack, but not both.
1206   if (LHS && RHS &&
1207       LHS->containsUnexpandedParameterPack() ==
1208           RHS->containsUnexpandedParameterPack()) {
1209     DiscardOperands();
1210     return Diag(EllipsisLoc,
1211                 LHS->containsUnexpandedParameterPack()
1212                     ? diag::err_fold_expression_packs_both_sides
1213                     : diag::err_pack_expansion_without_parameter_packs)
1214         << LHS->getSourceRange() << RHS->getSourceRange();
1215   }
1216
1217   // [expr.prim.fold]p2:
1218   //   In a unary fold, the cast-expression shall contain an unexpanded
1219   //   parameter pack.
1220   if (!LHS || !RHS) {
1221     Expr *Pack = LHS ? LHS : RHS;
1222     assert(Pack && "fold expression with neither LHS nor RHS");
1223     if (!Pack->containsUnexpandedParameterPack()) {
1224       DiscardOperands();
1225       return Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
1226              << Pack->getSourceRange();
1227     }
1228   }
1229
1230   BinaryOperatorKind Opc = ConvertTokenKindToBinaryOpcode(Operator);
1231
1232   // Perform first-phase name lookup now.
1233   UnresolvedLookupExpr *ULE = nullptr;
1234   {
1235     UnresolvedSet<16> Functions;
1236     LookupBinOp(S, EllipsisLoc, Opc, Functions);
1237     if (!Functions.empty()) {
1238       DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(
1239           BinaryOperator::getOverloadedOperator(Opc));
1240       ExprResult Callee = CreateUnresolvedLookupExpr(
1241           /*NamingClass*/ nullptr, NestedNameSpecifierLoc(),
1242           DeclarationNameInfo(OpName, EllipsisLoc), Functions);
1243       if (Callee.isInvalid())
1244         return ExprError();
1245       ULE = cast<UnresolvedLookupExpr>(Callee.get());
1246     }
1247   }
1248
1249   return BuildCXXFoldExpr(ULE, LParenLoc, LHS, Opc, EllipsisLoc, RHS, RParenLoc,
1250                           std::nullopt);
1251 }
1252
1253 ExprResult Sema::BuildCXXFoldExpr(UnresolvedLookupExpr *Callee,
1254                                   SourceLocation LParenLoc, Expr *LHS,
1255                                   BinaryOperatorKind Operator,
1256                                   SourceLocation EllipsisLoc, Expr *RHS,
1257                                   SourceLocation RParenLoc,
1258                                   std::optional<unsigned> NumExpansions) {
1259   return new (Context)
1260       CXXFoldExpr(Context.DependentTy, Callee, LParenLoc, LHS, Operator,
1261                   EllipsisLoc, RHS, RParenLoc, NumExpansions);
1262 }
1263
1264 ExprResult Sema::BuildEmptyCXXFoldExpr(SourceLocation EllipsisLoc,
1265                                        BinaryOperatorKind Operator) {
1266   // [temp.variadic]p9:
1267   //   If N is zero for a unary fold-expression, the value of the expression is
1268   //       &&  ->  true
1269   //       ||  ->  false
1270   //       ,   ->  void()
1271   //   if the operator is not listed [above], the instantiation is ill-formed.
1272   //
1273   // Note that we need to use something like int() here, not merely 0, to
1274   // prevent the result from being a null pointer constant.
1275   QualType ScalarType;
1276   switch (Operator) {
1277   case BO_LOr:
1278     return ActOnCXXBoolLiteral(EllipsisLoc, tok::kw_false);
1279   case BO_LAnd:
1280     return ActOnCXXBoolLiteral(EllipsisLoc, tok::kw_true);
1281   case BO_Comma:
1282     ScalarType = Context.VoidTy;
1283     break;
1284
1285   default:
1286     return Diag(EllipsisLoc, diag::err_fold_expression_empty)
1287         << BinaryOperator::getOpcodeStr(Operator);
1288   }
1289
1290   return new (Context) CXXScalarValueInitExpr(
1291       ScalarType, Context.getTrivialTypeSourceInfo(ScalarType, EllipsisLoc),
1292       EllipsisLoc);
1293 }