xref: /freebsd/contrib/llvm-project/clang/lib/AST/ExprClassification.cpp (revision 9f23cbd6cae82fd77edfad7173432fa8dccd0a95)
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   case Expr::CXXParenListInitExprClass:
450     if (isa<ArrayType>(E->getType()))
451       return Cl::CL_ArrayTemporary;
452     return Cl::CL_ClassTemporary;
453   }
454 
455   llvm_unreachable("unhandled expression kind in classification");
456 }
457 
458 /// ClassifyDecl - Return the classification of an expression referencing the
459 /// given declaration.
460 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D) {
461   // C++ [expr.prim.general]p6: The result is an lvalue if the entity is a
462   //   function, variable, or data member and a prvalue otherwise.
463   // In C, functions are not lvalues.
464   // In addition, NonTypeTemplateParmDecl derives from VarDecl but isn't an
465   // lvalue unless it's a reference type (C++ [temp.param]p6), so we need to
466   // special-case this.
467 
468   if (isa<CXXMethodDecl>(D) && cast<CXXMethodDecl>(D)->isInstance())
469     return Cl::CL_MemberFunction;
470 
471   bool islvalue;
472   if (const auto *NTTParm = dyn_cast<NonTypeTemplateParmDecl>(D))
473     islvalue = NTTParm->getType()->isReferenceType() ||
474                NTTParm->getType()->isRecordType();
475   else
476     islvalue =
477         isa<VarDecl, FieldDecl, IndirectFieldDecl, BindingDecl, MSGuidDecl,
478             UnnamedGlobalConstantDecl, TemplateParamObjectDecl>(D) ||
479         (Ctx.getLangOpts().CPlusPlus &&
480          (isa<FunctionDecl, MSPropertyDecl, FunctionTemplateDecl>(D)));
481 
482   return islvalue ? Cl::CL_LValue : Cl::CL_PRValue;
483 }
484 
485 /// ClassifyUnnamed - Return the classification of an expression yielding an
486 /// unnamed value of the given type. This applies in particular to function
487 /// calls and casts.
488 static Cl::Kinds ClassifyUnnamed(ASTContext &Ctx, QualType T) {
489   // In C, function calls are always rvalues.
490   if (!Ctx.getLangOpts().CPlusPlus) return Cl::CL_PRValue;
491 
492   // C++ [expr.call]p10: A function call is an lvalue if the result type is an
493   //   lvalue reference type or an rvalue reference to function type, an xvalue
494   //   if the result type is an rvalue reference to object type, and a prvalue
495   //   otherwise.
496   if (T->isLValueReferenceType())
497     return Cl::CL_LValue;
498   const auto *RV = T->getAs<RValueReferenceType>();
499   if (!RV) // Could still be a class temporary, though.
500     return ClassifyTemporary(T);
501 
502   return RV->getPointeeType()->isFunctionType() ? Cl::CL_LValue : Cl::CL_XValue;
503 }
504 
505 static Cl::Kinds ClassifyMemberExpr(ASTContext &Ctx, const MemberExpr *E) {
506   if (E->getType() == Ctx.UnknownAnyTy)
507     return (isa<FunctionDecl>(E->getMemberDecl())
508               ? Cl::CL_PRValue : Cl::CL_LValue);
509 
510   // Handle C first, it's easier.
511   if (!Ctx.getLangOpts().CPlusPlus) {
512     // C99 6.5.2.3p3
513     // For dot access, the expression is an lvalue if the first part is. For
514     // arrow access, it always is an lvalue.
515     if (E->isArrow())
516       return Cl::CL_LValue;
517     // ObjC property accesses are not lvalues, but get special treatment.
518     Expr *Base = E->getBase()->IgnoreParens();
519     if (isa<ObjCPropertyRefExpr>(Base))
520       return Cl::CL_SubObjCPropertySetting;
521     return ClassifyInternal(Ctx, Base);
522   }
523 
524   NamedDecl *Member = E->getMemberDecl();
525   // C++ [expr.ref]p3: E1->E2 is converted to the equivalent form (*(E1)).E2.
526   // C++ [expr.ref]p4: If E2 is declared to have type "reference to T", then
527   //   E1.E2 is an lvalue.
528   if (const auto *Value = dyn_cast<ValueDecl>(Member))
529     if (Value->getType()->isReferenceType())
530       return Cl::CL_LValue;
531 
532   //   Otherwise, one of the following rules applies.
533   //   -- If E2 is a static member [...] then E1.E2 is an lvalue.
534   if (isa<VarDecl>(Member) && Member->getDeclContext()->isRecord())
535     return Cl::CL_LValue;
536 
537   //   -- If E2 is a non-static data member [...]. If E1 is an lvalue, then
538   //      E1.E2 is an lvalue; if E1 is an xvalue, then E1.E2 is an xvalue;
539   //      otherwise, it is a prvalue.
540   if (isa<FieldDecl>(Member)) {
541     // *E1 is an lvalue
542     if (E->isArrow())
543       return Cl::CL_LValue;
544     Expr *Base = E->getBase()->IgnoreParenImpCasts();
545     if (isa<ObjCPropertyRefExpr>(Base))
546       return Cl::CL_SubObjCPropertySetting;
547     return ClassifyInternal(Ctx, E->getBase());
548   }
549 
550   //   -- If E2 is a [...] member function, [...]
551   //      -- If it refers to a static member function [...], then E1.E2 is an
552   //         lvalue; [...]
553   //      -- Otherwise [...] E1.E2 is a prvalue.
554   if (const auto *Method = dyn_cast<CXXMethodDecl>(Member))
555     return Method->isStatic() ? Cl::CL_LValue : Cl::CL_MemberFunction;
556 
557   //   -- If E2 is a member enumerator [...], the expression E1.E2 is a prvalue.
558   // So is everything else we haven't handled yet.
559   return Cl::CL_PRValue;
560 }
561 
562 static Cl::Kinds ClassifyBinaryOp(ASTContext &Ctx, const BinaryOperator *E) {
563   assert(Ctx.getLangOpts().CPlusPlus &&
564          "This is only relevant for C++.");
565   // C++ [expr.ass]p1: All [...] return an lvalue referring to the left operand.
566   // Except we override this for writes to ObjC properties.
567   if (E->isAssignmentOp())
568     return (E->getLHS()->getObjectKind() == OK_ObjCProperty
569               ? Cl::CL_PRValue : Cl::CL_LValue);
570 
571   // C++ [expr.comma]p1: the result is of the same value category as its right
572   //   operand, [...].
573   if (E->getOpcode() == BO_Comma)
574     return ClassifyInternal(Ctx, E->getRHS());
575 
576   // C++ [expr.mptr.oper]p6: The result of a .* expression whose second operand
577   //   is a pointer to a data member is of the same value category as its first
578   //   operand.
579   if (E->getOpcode() == BO_PtrMemD)
580     return (E->getType()->isFunctionType() ||
581             E->hasPlaceholderType(BuiltinType::BoundMember))
582              ? Cl::CL_MemberFunction
583              : ClassifyInternal(Ctx, E->getLHS());
584 
585   // C++ [expr.mptr.oper]p6: The result of an ->* expression is an lvalue if its
586   //   second operand is a pointer to data member and a prvalue otherwise.
587   if (E->getOpcode() == BO_PtrMemI)
588     return (E->getType()->isFunctionType() ||
589             E->hasPlaceholderType(BuiltinType::BoundMember))
590              ? Cl::CL_MemberFunction
591              : Cl::CL_LValue;
592 
593   // All other binary operations are prvalues.
594   return Cl::CL_PRValue;
595 }
596 
597 static Cl::Kinds ClassifyConditional(ASTContext &Ctx, const Expr *True,
598                                      const Expr *False) {
599   assert(Ctx.getLangOpts().CPlusPlus &&
600          "This is only relevant for C++.");
601 
602   // C++ [expr.cond]p2
603   //   If either the second or the third operand has type (cv) void,
604   //   one of the following shall hold:
605   if (True->getType()->isVoidType() || False->getType()->isVoidType()) {
606     // The second or the third operand (but not both) is a (possibly
607     // parenthesized) throw-expression; the result is of the [...] value
608     // category of the other.
609     bool TrueIsThrow = isa<CXXThrowExpr>(True->IgnoreParenImpCasts());
610     bool FalseIsThrow = isa<CXXThrowExpr>(False->IgnoreParenImpCasts());
611     if (const Expr *NonThrow = TrueIsThrow ? (FalseIsThrow ? nullptr : False)
612                                            : (FalseIsThrow ? True : nullptr))
613       return ClassifyInternal(Ctx, NonThrow);
614 
615     //   [Otherwise] the result [...] is a prvalue.
616     return Cl::CL_PRValue;
617   }
618 
619   // Note that at this point, we have already performed all conversions
620   // according to [expr.cond]p3.
621   // C++ [expr.cond]p4: If the second and third operands are glvalues of the
622   //   same value category [...], the result is of that [...] value category.
623   // C++ [expr.cond]p5: Otherwise, the result is a prvalue.
624   Cl::Kinds LCl = ClassifyInternal(Ctx, True),
625             RCl = ClassifyInternal(Ctx, False);
626   return LCl == RCl ? LCl : Cl::CL_PRValue;
627 }
628 
629 static Cl::ModifiableType IsModifiable(ASTContext &Ctx, const Expr *E,
630                                        Cl::Kinds Kind, SourceLocation &Loc) {
631   // As a general rule, we only care about lvalues. But there are some rvalues
632   // for which we want to generate special results.
633   if (Kind == Cl::CL_PRValue) {
634     // For the sake of better diagnostics, we want to specifically recognize
635     // use of the GCC cast-as-lvalue extension.
636     if (const auto *CE = dyn_cast<ExplicitCastExpr>(E->IgnoreParens())) {
637       if (CE->getSubExpr()->IgnoreParenImpCasts()->isLValue()) {
638         Loc = CE->getExprLoc();
639         return Cl::CM_LValueCast;
640       }
641     }
642   }
643   if (Kind != Cl::CL_LValue)
644     return Cl::CM_RValue;
645 
646   // This is the lvalue case.
647   // Functions are lvalues in C++, but not modifiable. (C++ [basic.lval]p6)
648   if (Ctx.getLangOpts().CPlusPlus && E->getType()->isFunctionType())
649     return Cl::CM_Function;
650 
651   // Assignment to a property in ObjC is an implicit setter access. But a
652   // setter might not exist.
653   if (const auto *Expr = dyn_cast<ObjCPropertyRefExpr>(E)) {
654     if (Expr->isImplicitProperty() &&
655         Expr->getImplicitPropertySetter() == nullptr)
656       return Cl::CM_NoSetterProperty;
657   }
658 
659   CanQualType CT = Ctx.getCanonicalType(E->getType());
660   // Const stuff is obviously not modifiable.
661   if (CT.isConstQualified())
662     return Cl::CM_ConstQualified;
663   if (Ctx.getLangOpts().OpenCL &&
664       CT.getQualifiers().getAddressSpace() == LangAS::opencl_constant)
665     return Cl::CM_ConstAddrSpace;
666 
667   // Arrays are not modifiable, only their elements are.
668   if (CT->isArrayType())
669     return Cl::CM_ArrayType;
670   // Incomplete types are not modifiable.
671   if (CT->isIncompleteType())
672     return Cl::CM_IncompleteType;
673 
674   // Records with any const fields (recursively) are not modifiable.
675   if (const RecordType *R = CT->getAs<RecordType>())
676     if (R->hasConstFields())
677       return Cl::CM_ConstQualifiedField;
678 
679   return Cl::CM_Modifiable;
680 }
681 
682 Expr::LValueClassification Expr::ClassifyLValue(ASTContext &Ctx) const {
683   Classification VC = Classify(Ctx);
684   switch (VC.getKind()) {
685   case Cl::CL_LValue: return LV_Valid;
686   case Cl::CL_XValue: return LV_InvalidExpression;
687   case Cl::CL_Function: return LV_NotObjectType;
688   case Cl::CL_Void: return LV_InvalidExpression;
689   case Cl::CL_AddressableVoid: return LV_IncompleteVoidType;
690   case Cl::CL_DuplicateVectorComponents: return LV_DuplicateVectorComponents;
691   case Cl::CL_MemberFunction: return LV_MemberFunction;
692   case Cl::CL_SubObjCPropertySetting: return LV_SubObjCPropertySetting;
693   case Cl::CL_ClassTemporary: return LV_ClassTemporary;
694   case Cl::CL_ArrayTemporary: return LV_ArrayTemporary;
695   case Cl::CL_ObjCMessageRValue: return LV_InvalidMessageExpression;
696   case Cl::CL_PRValue: return LV_InvalidExpression;
697   }
698   llvm_unreachable("Unhandled kind");
699 }
700 
701 Expr::isModifiableLvalueResult
702 Expr::isModifiableLvalue(ASTContext &Ctx, SourceLocation *Loc) const {
703   SourceLocation dummy;
704   Classification VC = ClassifyModifiable(Ctx, Loc ? *Loc : dummy);
705   switch (VC.getKind()) {
706   case Cl::CL_LValue: break;
707   case Cl::CL_XValue: return MLV_InvalidExpression;
708   case Cl::CL_Function: return MLV_NotObjectType;
709   case Cl::CL_Void: return MLV_InvalidExpression;
710   case Cl::CL_AddressableVoid: return MLV_IncompleteVoidType;
711   case Cl::CL_DuplicateVectorComponents: return MLV_DuplicateVectorComponents;
712   case Cl::CL_MemberFunction: return MLV_MemberFunction;
713   case Cl::CL_SubObjCPropertySetting: return MLV_SubObjCPropertySetting;
714   case Cl::CL_ClassTemporary: return MLV_ClassTemporary;
715   case Cl::CL_ArrayTemporary: return MLV_ArrayTemporary;
716   case Cl::CL_ObjCMessageRValue: return MLV_InvalidMessageExpression;
717   case Cl::CL_PRValue:
718     return VC.getModifiable() == Cl::CM_LValueCast ?
719       MLV_LValueCast : MLV_InvalidExpression;
720   }
721   assert(VC.getKind() == Cl::CL_LValue && "Unhandled kind");
722   switch (VC.getModifiable()) {
723   case Cl::CM_Untested: llvm_unreachable("Did not test modifiability");
724   case Cl::CM_Modifiable: return MLV_Valid;
725   case Cl::CM_RValue: llvm_unreachable("CM_RValue and CL_LValue don't match");
726   case Cl::CM_Function: return MLV_NotObjectType;
727   case Cl::CM_LValueCast:
728     llvm_unreachable("CM_LValueCast and CL_LValue don't match");
729   case Cl::CM_NoSetterProperty: return MLV_NoSetterProperty;
730   case Cl::CM_ConstQualified: return MLV_ConstQualified;
731   case Cl::CM_ConstQualifiedField: return MLV_ConstQualifiedField;
732   case Cl::CM_ConstAddrSpace: return MLV_ConstAddrSpace;
733   case Cl::CM_ArrayType: return MLV_ArrayType;
734   case Cl::CM_IncompleteType: return MLV_IncompleteType;
735   }
736   llvm_unreachable("Unhandled modifiable type");
737 }
738