clang/lib/AST/ExprClassification.cpp

   1 //===- ExprClassification.cpp - Expression AST Node Implementation --------===//
   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 //
   9 // This file implements Expr::classify.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "clang/AST/Expr.h"
  14 #include "clang/AST/ASTContext.h"
  15 #include "clang/AST/DeclCXX.h"
  16 #include "clang/AST/DeclObjC.h"
  17 #include "clang/AST/DeclTemplate.h"
  18 #include "clang/AST/ExprCXX.h"
  19 #include "clang/AST/ExprObjC.h"
  20 #include "llvm/Support/ErrorHandling.h"
  21
  22 using namespace clang;
  23
  24 using Cl = Expr::Classification;
  25
  26 static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E);
  27 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D);
  28 static Cl::Kinds ClassifyUnnamed(ASTContext &Ctx, QualType T);
  29 static Cl::Kinds ClassifyMemberExpr(ASTContext &Ctx, const MemberExpr *E);
  30 static Cl::Kinds ClassifyBinaryOp(ASTContext &Ctx, const BinaryOperator *E);
  31 static Cl::Kinds ClassifyConditional(ASTContext &Ctx,
  32                                      const Expr *trueExpr,
  33                                      const Expr *falseExpr);
  34 static Cl::ModifiableType IsModifiable(ASTContext &Ctx, const Expr *E,
  35                                        Cl::Kinds Kind, SourceLocation &Loc);
  36
  37 Cl Expr::ClassifyImpl(ASTContext &Ctx, SourceLocation *Loc) const {
  38   assert(!TR->isReferenceType() && "Expressions can't have reference type.");
  39
  40   Cl::Kinds kind = ClassifyInternal(Ctx, this);
  41   // C99 6.3.2.1: An lvalue is an expression with an object type or an
  42   //   incomplete type other than void.
  43   if (!Ctx.getLangOpts().CPlusPlus) {
  44     // Thus, no functions.
  45     if (TR->isFunctionType() || TR == Ctx.OverloadTy)
  46       kind = Cl::CL_Function;
  47     // No void either, but qualified void is OK because it is "other than void".
  48     // Void "lvalues" are classified as addressable void values, which are void
  49     // expressions whose address can be taken.
  50     else if (TR->isVoidType() && !TR.hasQualifiers())
  51       kind = (kind == Cl::CL_LValue ? Cl::CL_AddressableVoid : Cl::CL_Void);
  52   }
  53
  54   // Enable this assertion for testing.
  55   switch (kind) {
  56   case Cl::CL_LValue:
  57     assert(isLValue());
  58     break;
  59   case Cl::CL_XValue:
  60     assert(isXValue());
  61     break;
  62   case Cl::CL_Function:
  63   case Cl::CL_Void:
  64   case Cl::CL_AddressableVoid:
  65   case Cl::CL_DuplicateVectorComponents:
  66   case Cl::CL_MemberFunction:
  67   case Cl::CL_SubObjCPropertySetting:
  68   case Cl::CL_ClassTemporary:
  69   case Cl::CL_ArrayTemporary:
  70   case Cl::CL_ObjCMessageRValue:
  71   case Cl::CL_PRValue:
  72     assert(isPRValue());
  73     break;
  74   }
  75
  76   Cl::ModifiableType modifiable = Cl::CM_Untested;
  77   if (Loc)
  78     modifiable = IsModifiable(Ctx, this, kind, *Loc);
  79   return Classification(kind, modifiable);
  80 }
  81
  82 /// Classify an expression which creates a temporary, based on its type.
  83 static Cl::Kinds ClassifyTemporary(QualType T) {
  84   if (T->isRecordType())
  85     return Cl::CL_ClassTemporary;
  86   if (T->isArrayType())
  87     return Cl::CL_ArrayTemporary;
  88
  89   // No special classification: these don't behave differently from normal
  90   // prvalues.
  91   return Cl::CL_PRValue;
  92 }
  93
  94 static Cl::Kinds ClassifyExprValueKind(const LangOptions &Lang,
  95                                        const Expr *E,
  96                                        ExprValueKind Kind) {
  97   switch (Kind) {
  98   case VK_PRValue:
  99     return Lang.CPlusPlus ? ClassifyTemporary(E->getType()) : Cl::CL_PRValue;
 100   case VK_LValue:
 101     return Cl::CL_LValue;
 102   case VK_XValue:
 103     return Cl::CL_XValue;
 104   }
 105   llvm_unreachable("Invalid value category of implicit cast.");
 106 }
 107
 108 static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
 109   // This function takes the first stab at classifying expressions.
 110   const LangOptions &Lang = Ctx.getLangOpts();
 111
 112   switch (E->getStmtClass()) {
 113   case Stmt::NoStmtClass:
 114 #define ABSTRACT_STMT(Kind)
 115 #define STMT(Kind, Base) case Expr::Kind##Class:
 116 #define EXPR(Kind, Base)
 117 #include "clang/AST/StmtNodes.inc"
 118     llvm_unreachable("cannot classify a statement");
 119
 120     // First come the expressions that are always lvalues, unconditionally.
 121   case Expr::ObjCIsaExprClass:
 122     // C++ [expr.prim.general]p1: A string literal is an lvalue.
 123   case Expr::StringLiteralClass:
 124     // @encode is equivalent to its string
 125   case Expr::ObjCEncodeExprClass:
 126     // __func__ and friends are too.
 127   case Expr::PredefinedExprClass:
 128     // Property references are lvalues
 129   case Expr::ObjCSubscriptRefExprClass:
 130   case Expr::ObjCPropertyRefExprClass:
 131     // C++ [expr.typeid]p1: The result of a typeid expression is an lvalue of...
 132   case Expr::CXXTypeidExprClass:
 133   case Expr::CXXUuidofExprClass:
 134     // Unresolved lookups and uncorrected typos get classified as lvalues.
 135     // FIXME: Is this wise? Should they get their own kind?
 136   case Expr::UnresolvedLookupExprClass:
 137   case Expr::UnresolvedMemberExprClass:
 138   case Expr::TypoExprClass:
 139   case Expr::DependentCoawaitExprClass:
 140   case Expr::CXXDependentScopeMemberExprClass:
 141   case Expr::DependentScopeDeclRefExprClass:
 142     // ObjC instance variables are lvalues
 143     // FIXME: ObjC++0x might have different rules
 144   case Expr::ObjCIvarRefExprClass:
 145   case Expr::FunctionParmPackExprClass:
 146   case Expr::MSPropertyRefExprClass:
 147   case Expr::MSPropertySubscriptExprClass:
 148   case Expr::OMPArraySectionExprClass:
 149   case Expr::OMPArrayShapingExprClass:
 150   case Expr::OMPIteratorExprClass:
 151     return Cl::CL_LValue;
 152
 153     // C99 6.5.2.5p5 says that compound literals are lvalues.
 154     // In C++, they're prvalue temporaries, except for file-scope arrays.
 155   case Expr::CompoundLiteralExprClass:
 156     return !E->isLValue() ? ClassifyTemporary(E->getType()) : Cl::CL_LValue;
 157
 158     // Expressions that are prvalues.
 159   case Expr::CXXBoolLiteralExprClass:
 160   case Expr::CXXPseudoDestructorExprClass:
 161   case Expr::UnaryExprOrTypeTraitExprClass:
 162   case Expr::CXXNewExprClass:
 163   case Expr::CXXNullPtrLiteralExprClass:
 164   case Expr::ImaginaryLiteralClass:
 165   case Expr::GNUNullExprClass:
 166   case Expr::OffsetOfExprClass:
 167   case Expr::CXXThrowExprClass:
 168   case Expr::ShuffleVectorExprClass:
 169   case Expr::ConvertVectorExprClass:
 170   case Expr::IntegerLiteralClass:
 171   case Expr::FixedPointLiteralClass:
 172   case Expr::CharacterLiteralClass:
 173   case Expr::AddrLabelExprClass:
 174   case Expr::CXXDeleteExprClass:
 175   case Expr::ImplicitValueInitExprClass:
 176   case Expr::BlockExprClass:
 177   case Expr::FloatingLiteralClass:
 178   case Expr::CXXNoexceptExprClass:
 179   case Expr::CXXScalarValueInitExprClass:
 180   case Expr::TypeTraitExprClass:
 181   case Expr::ArrayTypeTraitExprClass:
 182   case Expr::ExpressionTraitExprClass:
 183   case Expr::ObjCSelectorExprClass:
 184   case Expr::ObjCProtocolExprClass:
 185   case Expr::ObjCStringLiteralClass:
 186   case Expr::ObjCBoxedExprClass:
 187   case Expr::ObjCArrayLiteralClass:
 188   case Expr::ObjCDictionaryLiteralClass:
 189   case Expr::ObjCBoolLiteralExprClass:
 190   case Expr::ObjCAvailabilityCheckExprClass:
 191   case Expr::ParenListExprClass:
 192   case Expr::SizeOfPackExprClass:
 193   case Expr::SubstNonTypeTemplateParmPackExprClass:
 194   case Expr::AsTypeExprClass:
 195   case Expr::ObjCIndirectCopyRestoreExprClass:
 196   case Expr::AtomicExprClass:
 197   case Expr::CXXFoldExprClass:
 198   case Expr::ArrayInitLoopExprClass:
 199   case Expr::ArrayInitIndexExprClass:
 200   case Expr::NoInitExprClass:
 201   case Expr::DesignatedInitUpdateExprClass:
 202   case Expr::SourceLocExprClass:
 203   case Expr::ConceptSpecializationExprClass:
 204   case Expr::RequiresExprClass:
 205     return Cl::CL_PRValue;
 206
 207   // Make HLSL this reference-like
 208   case Expr::CXXThisExprClass:
 209     return Lang.HLSL ? Cl::CL_LValue : Cl::CL_PRValue;
 210
 211   case Expr::ConstantExprClass:
 212     return ClassifyInternal(Ctx, cast<ConstantExpr>(E)->getSubExpr());
 213
 214     // Next come the complicated cases.
 215   case Expr::SubstNonTypeTemplateParmExprClass:
 216     return ClassifyInternal(Ctx,
 217                  cast<SubstNonTypeTemplateParmExpr>(E)->getReplacement());
 218
 219     // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 220     // C++11 (DR1213): in the case of an array operand, the result is an lvalue
 221     //                 if that operand is an lvalue and an xvalue otherwise.
 222     // Subscripting vector types is more like member access.
 223   case Expr::ArraySubscriptExprClass:
 224     if (cast<ArraySubscriptExpr>(E)->getBase()->getType()->isVectorType())
 225       return ClassifyInternal(Ctx, cast<ArraySubscriptExpr>(E)->getBase());
 226     if (Lang.CPlusPlus11) {
 227       // Step over the array-to-pointer decay if present, but not over the
 228       // temporary materialization.
 229       auto *Base = cast<ArraySubscriptExpr>(E)->getBase()->IgnoreImpCasts();
 230       if (Base->getType()->isArrayType())
 231         return ClassifyInternal(Ctx, Base);
 232     }
 233     return Cl::CL_LValue;
 234
 235   // Subscripting matrix types behaves like member accesses.
 236   case Expr::MatrixSubscriptExprClass:
 237     return ClassifyInternal(Ctx, cast<MatrixSubscriptExpr>(E)->getBase());
 238
 239     // C++ [expr.prim.general]p3: The result is an lvalue if the entity is a
 240     //   function or variable and a prvalue otherwise.
 241   case Expr::DeclRefExprClass:
 242     if (E->getType() == Ctx.UnknownAnyTy)
 243       return isa<FunctionDecl>(cast<DeclRefExpr>(E)->getDecl())
 244                ? Cl::CL_PRValue : Cl::CL_LValue;
 245     return ClassifyDecl(Ctx, cast<DeclRefExpr>(E)->getDecl());
 246
 247     // Member access is complex.
 248   case Expr::MemberExprClass:
 249     return ClassifyMemberExpr(Ctx, cast<MemberExpr>(E));
 250
 251   case Expr::UnaryOperatorClass:
 252     switch (cast<UnaryOperator>(E)->getOpcode()) {
 253       // C++ [expr.unary.op]p1: The unary * operator performs indirection:
 254       //   [...] the result is an lvalue referring to the object or function
 255       //   to which the expression points.
 256     case UO_Deref:
 257       return Cl::CL_LValue;
 258
 259       // GNU extensions, simply look through them.
 260     case UO_Extension:
 261       return ClassifyInternal(Ctx, cast<UnaryOperator>(E)->getSubExpr());
 262
 263     // Treat _Real and _Imag basically as if they were member
 264     // expressions:  l-value only if the operand is a true l-value.
 265     case UO_Real:
 266     case UO_Imag: {
 267       const Expr *Op = cast<UnaryOperator>(E)->getSubExpr()->IgnoreParens();
 268       Cl::Kinds K = ClassifyInternal(Ctx, Op);
 269       if (K != Cl::CL_LValue) return K;
 270
 271       if (isa<ObjCPropertyRefExpr>(Op))
 272         return Cl::CL_SubObjCPropertySetting;
 273       return Cl::CL_LValue;
 274     }
 275
 276       // C++ [expr.pre.incr]p1: The result is the updated operand; it is an
 277       //   lvalue, [...]
 278       // Not so in C.
 279     case UO_PreInc:
 280     case UO_PreDec:
 281       return Lang.CPlusPlus ? Cl::CL_LValue : Cl::CL_PRValue;
 282
 283     default:
 284       return Cl::CL_PRValue;
 285     }
 286
 287   case Expr::RecoveryExprClass:
 288   case Expr::OpaqueValueExprClass:
 289     return ClassifyExprValueKind(Lang, E, E->getValueKind());
 290
 291     // Pseudo-object expressions can produce l-values with reference magic.
 292   case Expr::PseudoObjectExprClass:
 293     return ClassifyExprValueKind(Lang, E,
 294                                  cast<PseudoObjectExpr>(E)->getValueKind());
 295
 296     // Implicit casts are lvalues if they're lvalue casts. Other than that, we
 297     // only specifically record class temporaries.
 298   case Expr::ImplicitCastExprClass:
 299     return ClassifyExprValueKind(Lang, E, E->getValueKind());
 300
 301     // C++ [expr.prim.general]p4: The presence of parentheses does not affect
 302     //   whether the expression is an lvalue.
 303   case Expr::ParenExprClass:
 304     return ClassifyInternal(Ctx, cast<ParenExpr>(E)->getSubExpr());
 305
 306     // C11 6.5.1.1p4: [A generic selection] is an lvalue, a function designator,
 307     // or a void expression if its result expression is, respectively, an
 308     // lvalue, a function designator, or a void expression.
 309   case Expr::GenericSelectionExprClass:
 310     if (cast<GenericSelectionExpr>(E)->isResultDependent())
 311       return Cl::CL_PRValue;
 312     return ClassifyInternal(Ctx,cast<GenericSelectionExpr>(E)->getResultExpr());
 313
 314   case Expr::BinaryOperatorClass:
 315   case Expr::CompoundAssignOperatorClass:
 316     // C doesn't have any binary expressions that are lvalues.
 317     if (Lang.CPlusPlus)
 318       return ClassifyBinaryOp(Ctx, cast<BinaryOperator>(E));
 319     return Cl::CL_PRValue;
 320
 321   case Expr::CallExprClass:
 322   case Expr::CXXOperatorCallExprClass:
 323   case Expr::CXXMemberCallExprClass:
 324   case Expr::UserDefinedLiteralClass:
 325   case Expr::CUDAKernelCallExprClass:
 326     return ClassifyUnnamed(Ctx, cast<CallExpr>(E)->getCallReturnType(Ctx));
 327
 328   case Expr::CXXRewrittenBinaryOperatorClass:
 329     return ClassifyInternal(
 330         Ctx, cast<CXXRewrittenBinaryOperator>(E)->getSemanticForm());
 331
 332     // __builtin_choose_expr is equivalent to the chosen expression.
 333   case Expr::ChooseExprClass:
 334     return ClassifyInternal(Ctx, cast<ChooseExpr>(E)->getChosenSubExpr());
 335
 336     // Extended vector element access is an lvalue unless there are duplicates
 337     // in the shuffle expression.
 338   case Expr::ExtVectorElementExprClass:
 339     if (cast<ExtVectorElementExpr>(E)->containsDuplicateElements())
 340       return Cl::CL_DuplicateVectorComponents;
 341     if (cast<ExtVectorElementExpr>(E)->isArrow())
 342       return Cl::CL_LValue;
 343     return ClassifyInternal(Ctx, cast<ExtVectorElementExpr>(E)->getBase());
 344
 345     // Simply look at the actual default argument.
 346   case Expr::CXXDefaultArgExprClass:
 347     return ClassifyInternal(Ctx, cast<CXXDefaultArgExpr>(E)->getExpr());
 348
 349     // Same idea for default initializers.
 350   case Expr::CXXDefaultInitExprClass:
 351     return ClassifyInternal(Ctx, cast<CXXDefaultInitExpr>(E)->getExpr());
 352
 353     // Same idea for temporary binding.
 354   case Expr::CXXBindTemporaryExprClass:
 355     return ClassifyInternal(Ctx, cast<CXXBindTemporaryExpr>(E)->getSubExpr());
 356
 357     // And the cleanups guard.
 358   case Expr::ExprWithCleanupsClass:
 359     return ClassifyInternal(Ctx, cast<ExprWithCleanups>(E)->getSubExpr());
 360
 361     // Casts depend completely on the target type. All casts work the same.
 362   case Expr::CStyleCastExprClass:
 363   case Expr::CXXFunctionalCastExprClass:
 364   case Expr::CXXStaticCastExprClass:
 365   case Expr::CXXDynamicCastExprClass:
 366   case Expr::CXXReinterpretCastExprClass:
 367   case Expr::CXXConstCastExprClass:
 368   case Expr::CXXAddrspaceCastExprClass:
 369   case Expr::ObjCBridgedCastExprClass:
 370   case Expr::BuiltinBitCastExprClass:
 371     // Only in C++ can casts be interesting at all.
 372     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
 373     return ClassifyUnnamed(Ctx, cast<ExplicitCastExpr>(E)->getTypeAsWritten());
 374
 375   case Expr::CXXUnresolvedConstructExprClass:
 376     return ClassifyUnnamed(Ctx,
 377                       cast<CXXUnresolvedConstructExpr>(E)->getTypeAsWritten());
 378
 379   case Expr::BinaryConditionalOperatorClass: {
 380     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
 381     const auto *co = cast<BinaryConditionalOperator>(E);
 382     return ClassifyConditional(Ctx, co->getTrueExpr(), co->getFalseExpr());
 383   }
 384
 385   case Expr::ConditionalOperatorClass: {
 386     // Once again, only C++ is interesting.
 387     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
 388     const auto *co = cast<ConditionalOperator>(E);
 389     return ClassifyConditional(Ctx, co->getTrueExpr(), co->getFalseExpr());
 390   }
 391
 392     // ObjC message sends are effectively function calls, if the target function
 393     // is known.
 394   case Expr::ObjCMessageExprClass:
 395     if (const ObjCMethodDecl *Method =
 396           cast<ObjCMessageExpr>(E)->getMethodDecl()) {
 397       Cl::Kinds kind = ClassifyUnnamed(Ctx, Method->getReturnType());
 398       return (kind == Cl::CL_PRValue) ? Cl::CL_ObjCMessageRValue : kind;
 399     }
 400     return Cl::CL_PRValue;
 401
 402     // Some C++ expressions are always class temporaries.
 403   case Expr::CXXConstructExprClass:
 404   case Expr::CXXInheritedCtorInitExprClass:
 405   case Expr::CXXTemporaryObjectExprClass:
 406   case Expr::LambdaExprClass:
 407   case Expr::CXXStdInitializerListExprClass:
 408     return Cl::CL_ClassTemporary;
 409
 410   case Expr::VAArgExprClass:
 411     return ClassifyUnnamed(Ctx, E->getType());
 412
 413   case Expr::DesignatedInitExprClass:
 414     return ClassifyInternal(Ctx, cast<DesignatedInitExpr>(E)->getInit());
 415
 416   case Expr::StmtExprClass: {
 417     const CompoundStmt *S = cast<StmtExpr>(E)->getSubStmt();
 418     if (const auto *LastExpr = dyn_cast_or_null<Expr>(S->body_back()))
 419       return ClassifyUnnamed(Ctx, LastExpr->getType());
 420     return Cl::CL_PRValue;
 421   }
 422
 423   case Expr::PackExpansionExprClass:
 424     return ClassifyInternal(Ctx, cast<PackExpansionExpr>(E)->getPattern());
 425
 426   case Expr::MaterializeTemporaryExprClass:
 427     return cast<MaterializeTemporaryExpr>(E)->isBoundToLvalueReference()
 428               ? Cl::CL_LValue
 429               : Cl::CL_XValue;
 430
 431   case Expr::InitListExprClass:
 432     // An init list can be an lvalue if it is bound to a reference and
 433     // contains only one element. In that case, we look at that element
 434     // for an exact classification. Init list creation takes care of the
 435     // value kind for us, so we only need to fine-tune.
 436     if (E->isPRValue())
 437       return ClassifyExprValueKind(Lang, E, E->getValueKind());
 438     assert(cast<InitListExpr>(E)->getNumInits() == 1 &&
 439            "Only 1-element init lists can be glvalues.");
 440     return ClassifyInternal(Ctx, cast<InitListExpr>(E)->getInit(0));
 441
 442   case Expr::CoawaitExprClass:
 443   case Expr::CoyieldExprClass:
 444     return ClassifyInternal(Ctx, cast<CoroutineSuspendExpr>(E)->getResumeExpr());
 445   case Expr::SYCLUniqueStableNameExprClass:
 446     return Cl::CL_PRValue;
 447     break;
 448   }
 449
 450   llvm_unreachable("unhandled expression kind in classification");
 451 }
 452
 453 /// ClassifyDecl - Return the classification of an expression referencing the
 454 /// given declaration.
 455 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D) {
 456   // C++ [expr.prim.general]p6: The result is an lvalue if the entity is a
 457   //   function, variable, or data member and a prvalue otherwise.
 458   // In C, functions are not lvalues.
 459   // In addition, NonTypeTemplateParmDecl derives from VarDecl but isn't an
 460   // lvalue unless it's a reference type (C++ [temp.param]p6), so we need to
 461   // special-case this.
 462
 463   if (isa<CXXMethodDecl>(D) && cast<CXXMethodDecl>(D)->isInstance())
 464     return Cl::CL_MemberFunction;
 465
 466   bool islvalue;
 467   if (const auto *NTTParm = dyn_cast<NonTypeTemplateParmDecl>(D))
 468     islvalue = NTTParm->getType()->isReferenceType() ||
 469                NTTParm->getType()->isRecordType();
 470   else
 471     islvalue =
 472         isa<VarDecl, FieldDecl, IndirectFieldDecl, BindingDecl, MSGuidDecl,
 473             UnnamedGlobalConstantDecl, TemplateParamObjectDecl>(D) ||
 474         (Ctx.getLangOpts().CPlusPlus &&
 475          (isa<FunctionDecl, MSPropertyDecl, FunctionTemplateDecl>(D)));
 476
 477   return islvalue ? Cl::CL_LValue : Cl::CL_PRValue;
 478 }
 479
 480 /// ClassifyUnnamed - Return the classification of an expression yielding an
 481 /// unnamed value of the given type. This applies in particular to function
 482 /// calls and casts.
 483 static Cl::Kinds ClassifyUnnamed(ASTContext &Ctx, QualType T) {
 484   // In C, function calls are always rvalues.
 485   if (!Ctx.getLangOpts().CPlusPlus) return Cl::CL_PRValue;
 486
 487   // C++ [expr.call]p10: A function call is an lvalue if the result type is an
 488   //   lvalue reference type or an rvalue reference to function type, an xvalue
 489   //   if the result type is an rvalue reference to object type, and a prvalue
 490   //   otherwise.
 491   if (T->isLValueReferenceType())
 492     return Cl::CL_LValue;
 493   const auto *RV = T->getAs<RValueReferenceType>();
 494   if (!RV) // Could still be a class temporary, though.
 495     return ClassifyTemporary(T);
 496
 497   return RV->getPointeeType()->isFunctionType() ? Cl::CL_LValue : Cl::CL_XValue;
 498 }
 499
 500 static Cl::Kinds ClassifyMemberExpr(ASTContext &Ctx, const MemberExpr *E) {
 501   if (E->getType() == Ctx.UnknownAnyTy)
 502     return (isa<FunctionDecl>(E->getMemberDecl())
 503               ? Cl::CL_PRValue : Cl::CL_LValue);
 504
 505   // Handle C first, it's easier.
 506   if (!Ctx.getLangOpts().CPlusPlus) {
 507     // C99 6.5.2.3p3
 508     // For dot access, the expression is an lvalue if the first part is. For
 509     // arrow access, it always is an lvalue.
 510     if (E->isArrow())
 511       return Cl::CL_LValue;
 512     // ObjC property accesses are not lvalues, but get special treatment.
 513     Expr *Base = E->getBase()->IgnoreParens();
 514     if (isa<ObjCPropertyRefExpr>(Base))
 515       return Cl::CL_SubObjCPropertySetting;
 516     return ClassifyInternal(Ctx, Base);
 517   }
 518
 519   NamedDecl *Member = E->getMemberDecl();
 520   // C++ [expr.ref]p3: E1->E2 is converted to the equivalent form (*(E1)).E2.
 521   // C++ [expr.ref]p4: If E2 is declared to have type "reference to T", then
 522   //   E1.E2 is an lvalue.
 523   if (const auto *Value = dyn_cast<ValueDecl>(Member))
 524     if (Value->getType()->isReferenceType())
 525       return Cl::CL_LValue;
 526
 527   //   Otherwise, one of the following rules applies.
 528   //   -- If E2 is a static member [...] then E1.E2 is an lvalue.
 529   if (isa<VarDecl>(Member) && Member->getDeclContext()->isRecord())
 530     return Cl::CL_LValue;
 531
 532   //   -- If E2 is a non-static data member [...]. If E1 is an lvalue, then
 533   //      E1.E2 is an lvalue; if E1 is an xvalue, then E1.E2 is an xvalue;
 534   //      otherwise, it is a prvalue.
 535   if (isa<FieldDecl>(Member)) {
 536     // *E1 is an lvalue
 537     if (E->isArrow())
 538       return Cl::CL_LValue;
 539     Expr *Base = E->getBase()->IgnoreParenImpCasts();
 540     if (isa<ObjCPropertyRefExpr>(Base))
 541       return Cl::CL_SubObjCPropertySetting;
 542     return ClassifyInternal(Ctx, E->getBase());
 543   }
 544
 545   //   -- If E2 is a [...] member function, [...]
 546   //      -- If it refers to a static member function [...], then E1.E2 is an
 547   //         lvalue; [...]
 548   //      -- Otherwise [...] E1.E2 is a prvalue.
 549   if (const auto *Method = dyn_cast<CXXMethodDecl>(Member))
 550     return Method->isStatic() ? Cl::CL_LValue : Cl::CL_MemberFunction;
 551
 552   //   -- If E2 is a member enumerator [...], the expression E1.E2 is a prvalue.
 553   // So is everything else we haven't handled yet.
 554   return Cl::CL_PRValue;
 555 }
 556
 557 static Cl::Kinds ClassifyBinaryOp(ASTContext &Ctx, const BinaryOperator *E) {
 558   assert(Ctx.getLangOpts().CPlusPlus &&
 559          "This is only relevant for C++.");
 560   // C++ [expr.ass]p1: All [...] return an lvalue referring to the left operand.
 561   // Except we override this for writes to ObjC properties.
 562   if (E->isAssignmentOp())
 563     return (E->getLHS()->getObjectKind() == OK_ObjCProperty
 564               ? Cl::CL_PRValue : Cl::CL_LValue);
 565
 566   // C++ [expr.comma]p1: the result is of the same value category as its right
 567   //   operand, [...].
 568   if (E->getOpcode() == BO_Comma)
 569     return ClassifyInternal(Ctx, E->getRHS());
 570
 571   // C++ [expr.mptr.oper]p6: The result of a .* expression whose second operand
 572   //   is a pointer to a data member is of the same value category as its first
 573   //   operand.
 574   if (E->getOpcode() == BO_PtrMemD)
 575     return (E->getType()->isFunctionType() ||
 576             E->hasPlaceholderType(BuiltinType::BoundMember))
 577              ? Cl::CL_MemberFunction
 578              : ClassifyInternal(Ctx, E->getLHS());
 579
 580   // C++ [expr.mptr.oper]p6: The result of an ->* expression is an lvalue if its
 581   //   second operand is a pointer to data member and a prvalue otherwise.
 582   if (E->getOpcode() == BO_PtrMemI)
 583     return (E->getType()->isFunctionType() ||
 584             E->hasPlaceholderType(BuiltinType::BoundMember))
 585              ? Cl::CL_MemberFunction
 586              : Cl::CL_LValue;
 587
 588   // All other binary operations are prvalues.
 589   return Cl::CL_PRValue;
 590 }
 591
 592 static Cl::Kinds ClassifyConditional(ASTContext &Ctx, const Expr *True,
 593                                      const Expr *False) {
 594   assert(Ctx.getLangOpts().CPlusPlus &&
 595          "This is only relevant for C++.");
 596
 597   // C++ [expr.cond]p2
 598   //   If either the second or the third operand has type (cv) void,
 599   //   one of the following shall hold:
 600   if (True->getType()->isVoidType() || False->getType()->isVoidType()) {
 601     // The second or the third operand (but not both) is a (possibly
 602     // parenthesized) throw-expression; the result is of the [...] value
 603     // category of the other.
 604     bool TrueIsThrow = isa<CXXThrowExpr>(True->IgnoreParenImpCasts());
 605     bool FalseIsThrow = isa<CXXThrowExpr>(False->IgnoreParenImpCasts());
 606     if (const Expr *NonThrow = TrueIsThrow ? (FalseIsThrow ? nullptr : False)
 607                                            : (FalseIsThrow ? True : nullptr))
 608       return ClassifyInternal(Ctx, NonThrow);
 609
 610     //   [Otherwise] the result [...] is a prvalue.
 611     return Cl::CL_PRValue;
 612   }
 613
 614   // Note that at this point, we have already performed all conversions
 615   // according to [expr.cond]p3.
 616   // C++ [expr.cond]p4: If the second and third operands are glvalues of the
 617   //   same value category [...], the result is of that [...] value category.
 618   // C++ [expr.cond]p5: Otherwise, the result is a prvalue.
 619   Cl::Kinds LCl = ClassifyInternal(Ctx, True),
 620             RCl = ClassifyInternal(Ctx, False);
 621   return LCl == RCl ? LCl : Cl::CL_PRValue;
 622 }
 623
 624 static Cl::ModifiableType IsModifiable(ASTContext &Ctx, const Expr *E,
 625                                        Cl::Kinds Kind, SourceLocation &Loc) {
 626   // As a general rule, we only care about lvalues. But there are some rvalues
 627   // for which we want to generate special results.
 628   if (Kind == Cl::CL_PRValue) {
 629     // For the sake of better diagnostics, we want to specifically recognize
 630     // use of the GCC cast-as-lvalue extension.
 631     if (const auto *CE = dyn_cast<ExplicitCastExpr>(E->IgnoreParens())) {
 632       if (CE->getSubExpr()->IgnoreParenImpCasts()->isLValue()) {
 633         Loc = CE->getExprLoc();
 634         return Cl::CM_LValueCast;
 635       }
 636     }
 637   }
 638   if (Kind != Cl::CL_LValue)
 639     return Cl::CM_RValue;
 640
 641   // This is the lvalue case.
 642   // Functions are lvalues in C++, but not modifiable. (C++ [basic.lval]p6)
 643   if (Ctx.getLangOpts().CPlusPlus && E->getType()->isFunctionType())
 644     return Cl::CM_Function;
 645
 646   // Assignment to a property in ObjC is an implicit setter access. But a
 647   // setter might not exist.
 648   if (const auto *Expr = dyn_cast<ObjCPropertyRefExpr>(E)) {
 649     if (Expr->isImplicitProperty() &&
 650         Expr->getImplicitPropertySetter() == nullptr)
 651       return Cl::CM_NoSetterProperty;
 652   }
 653
 654   CanQualType CT = Ctx.getCanonicalType(E->getType());
 655   // Const stuff is obviously not modifiable.
 656   if (CT.isConstQualified())
 657     return Cl::CM_ConstQualified;
 658   if (Ctx.getLangOpts().OpenCL &&
 659       CT.getQualifiers().getAddressSpace() == LangAS::opencl_constant)
 660     return Cl::CM_ConstAddrSpace;
 661
 662   // Arrays are not modifiable, only their elements are.
 663   if (CT->isArrayType())
 664     return Cl::CM_ArrayType;
 665   // Incomplete types are not modifiable.
 666   if (CT->isIncompleteType())
 667     return Cl::CM_IncompleteType;
 668
 669   // Records with any const fields (recursively) are not modifiable.
 670   if (const RecordType *R = CT->getAs<RecordType>())
 671     if (R->hasConstFields())
 672       return Cl::CM_ConstQualifiedField;
 673
 674   return Cl::CM_Modifiable;
 675 }
 676
 677 Expr::LValueClassification Expr::ClassifyLValue(ASTContext &Ctx) const {
 678   Classification VC = Classify(Ctx);
 679   switch (VC.getKind()) {
 680   case Cl::CL_LValue: return LV_Valid;
 681   case Cl::CL_XValue: return LV_InvalidExpression;
 682   case Cl::CL_Function: return LV_NotObjectType;
 683   case Cl::CL_Void: return LV_InvalidExpression;
 684   case Cl::CL_AddressableVoid: return LV_IncompleteVoidType;
 685   case Cl::CL_DuplicateVectorComponents: return LV_DuplicateVectorComponents;
 686   case Cl::CL_MemberFunction: return LV_MemberFunction;
 687   case Cl::CL_SubObjCPropertySetting: return LV_SubObjCPropertySetting;
 688   case Cl::CL_ClassTemporary: return LV_ClassTemporary;
 689   case Cl::CL_ArrayTemporary: return LV_ArrayTemporary;
 690   case Cl::CL_ObjCMessageRValue: return LV_InvalidMessageExpression;
 691   case Cl::CL_PRValue: return LV_InvalidExpression;
 692   }
 693   llvm_unreachable("Unhandled kind");
 694 }
 695
 696 Expr::isModifiableLvalueResult
 697 Expr::isModifiableLvalue(ASTContext &Ctx, SourceLocation *Loc) const {
 698   SourceLocation dummy;
 699   Classification VC = ClassifyModifiable(Ctx, Loc ? *Loc : dummy);
 700   switch (VC.getKind()) {
 701   case Cl::CL_LValue: break;
 702   case Cl::CL_XValue: return MLV_InvalidExpression;
 703   case Cl::CL_Function: return MLV_NotObjectType;
 704   case Cl::CL_Void: return MLV_InvalidExpression;
 705   case Cl::CL_AddressableVoid: return MLV_IncompleteVoidType;
 706   case Cl::CL_DuplicateVectorComponents: return MLV_DuplicateVectorComponents;
 707   case Cl::CL_MemberFunction: return MLV_MemberFunction;
 708   case Cl::CL_SubObjCPropertySetting: return MLV_SubObjCPropertySetting;
 709   case Cl::CL_ClassTemporary: return MLV_ClassTemporary;
 710   case Cl::CL_ArrayTemporary: return MLV_ArrayTemporary;
 711   case Cl::CL_ObjCMessageRValue: return MLV_InvalidMessageExpression;
 712   case Cl::CL_PRValue:
 713     return VC.getModifiable() == Cl::CM_LValueCast ?
 714       MLV_LValueCast : MLV_InvalidExpression;
 715   }
 716   assert(VC.getKind() == Cl::CL_LValue && "Unhandled kind");
 717   switch (VC.getModifiable()) {
 718   case Cl::CM_Untested: llvm_unreachable("Did not test modifiability");
 719   case Cl::CM_Modifiable: return MLV_Valid;
 720   case Cl::CM_RValue: llvm_unreachable("CM_RValue and CL_LValue don't match");
 721   case Cl::CM_Function: return MLV_NotObjectType;
 722   case Cl::CM_LValueCast:
 723     llvm_unreachable("CM_LValueCast and CL_LValue don't match");
 724   case Cl::CM_NoSetterProperty: return MLV_NoSetterProperty;
 725   case Cl::CM_ConstQualified: return MLV_ConstQualified;
 726   case Cl::CM_ConstQualifiedField: return MLV_ConstQualifiedField;
 727   case Cl::CM_ConstAddrSpace: return MLV_ConstAddrSpace;
 728   case Cl::CM_ArrayType: return MLV_ArrayType;
 729   case Cl::CM_IncompleteType: return MLV_IncompleteType;
 730   }
 731   llvm_unreachable("Unhandled modifiable type");
 732 }