[Clang] Introduce CXXTypeidExpr::hasNullCheck (#95718)

Used to implement CWG2191 where `typeid` for a polymorphic glvalue only
becomes potentially-throwing if the `typeid` operand was already
potentially throwing or a `nullptr` check was inserted:
https://cplusplus.github.io/CWG/issues/2191.html

Also change `Expr::hasSideEffects` for `CXXTypeidExpr` to check the
operand for side-effects instead of always reporting that there are
side-effects

Remove `IsDeref` parameter of `CGCXXABI::shouldTypeidBeNullChecked`
because it should never return `true` if `!IsDeref` (we shouldn't add a
null check that wasn't there in the first place)

clang/docs/ReleaseNotes.rst

@@ -268,6 +268,9 @@ Resolutions to C++ Defect Reports
 - Clang now requires a template argument list after a template keyword.
   (`CWG96: Syntactic disambiguation using the template keyword <https://cplusplus.github.io/CWG/issues/96.html>`_).
 
+- Clang now considers ``noexcept(typeid(expr))`` more carefully, instead of always assuming that ``std::bad_typeid`` can be thrown.
+  (`CWG2191: Incorrect result for noexcept(typeid(v)) <https://cplusplus.github.io/CWG/issues/2191.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/AST/ExprCXX.h

@@ -919,6 +919,10 @@ public:
         reinterpret_cast<Stmt **>(&const_cast<CXXTypeidExpr *>(this)->Operand);
     return const_child_range(begin, begin + 1);
   }
+
+  /// Whether this is of a form like "typeid(*ptr)" that can throw a
+  /// std::bad_typeid if a pointer is a null pointer ([expr.typeid]p2)
+  bool hasNullCheck() const;
 };
 
 /// A member reference to an MSPropertyDecl.

clang/lib/AST/Expr.cpp

@@ -3769,10 +3769,18 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
     break;
   }
 
-  case CXXTypeidExprClass:
-    // typeid might throw if its subexpression is potentially-evaluated, so has
-    // side-effects in that case whether or not its subexpression does.
-    return cast<CXXTypeidExpr>(this)->isPotentiallyEvaluated();
+  case CXXTypeidExprClass: {
+    const auto *TE = cast<CXXTypeidExpr>(this);
+    if (!TE->isPotentiallyEvaluated())
+      return false;
+
+    // If this type id expression can throw because of a null pointer, that is a
+    // side-effect independent of if the operand has a side-effect
+    if (IncludePossibleEffects && TE->hasNullCheck())
+      return true;
+
+    break;
+  }
 
   case CXXConstructExprClass:
   case CXXTemporaryObjectExprClass: {

clang/lib/AST/ExprCXX.cpp

@@ -166,6 +166,53 @@ QualType CXXTypeidExpr::getTypeOperand(ASTContext &Context) const {
       Operand.get<TypeSourceInfo *>()->getType().getNonReferenceType(), Quals);
 }
 
+static bool isGLValueFromPointerDeref(const Expr *E) {
+  E = E->IgnoreParens();
+
+  if (const auto *CE = dyn_cast<CastExpr>(E)) {
+    if (!CE->getSubExpr()->isGLValue())
+      return false;
+    return isGLValueFromPointerDeref(CE->getSubExpr());
+  }
+
+  if (const auto *OVE = dyn_cast<OpaqueValueExpr>(E))
+    return isGLValueFromPointerDeref(OVE->getSourceExpr());
+
+  if (const auto *BO = dyn_cast<BinaryOperator>(E))
+    if (BO->getOpcode() == BO_Comma)
+      return isGLValueFromPointerDeref(BO->getRHS());
+
+  if (const auto *ACO = dyn_cast<AbstractConditionalOperator>(E))
+    return isGLValueFromPointerDeref(ACO->getTrueExpr()) ||
+           isGLValueFromPointerDeref(ACO->getFalseExpr());
+
+  // C++11 [expr.sub]p1:
+  //   The expression E1[E2] is identical (by definition) to *((E1)+(E2))
+  if (isa<ArraySubscriptExpr>(E))
+    return true;
+
+  if (const auto *UO = dyn_cast<UnaryOperator>(E))
+    if (UO->getOpcode() == UO_Deref)
+      return true;
+
+  return false;
+}
+
+bool CXXTypeidExpr::hasNullCheck() const {
+  if (!isPotentiallyEvaluated())
+    return false;
+
+  // C++ [expr.typeid]p2:
+  //   If the glvalue expression is obtained by applying the unary * operator to
+  //   a pointer and the pointer is a null pointer value, the typeid expression
+  //   throws the std::bad_typeid exception.
+  //
+  // However, this paragraph's intent is not clear.  We choose a very generous
+  // interpretation which implores us to consider comma operators, conditional
+  // operators, parentheses and other such constructs.
+  return isGLValueFromPointerDeref(getExprOperand());
+}
+
 QualType CXXUuidofExpr::getTypeOperand(ASTContext &Context) const {
   assert(isTypeOperand() && "Cannot call getTypeOperand for __uuidof(expr)");
   Qualifiers Quals;

clang/lib/CodeGen/CGCXXABI.h

@@ -274,8 +274,7 @@ public:
   getAddrOfCXXCatchHandlerType(QualType Ty, QualType CatchHandlerType) = 0;
   virtual CatchTypeInfo getCatchAllTypeInfo();
 
-  virtual bool shouldTypeidBeNullChecked(bool IsDeref,
-                                         QualType SrcRecordTy) = 0;
+  virtual bool shouldTypeidBeNullChecked(QualType SrcRecordTy) = 0;
   virtual void EmitBadTypeidCall(CodeGenFunction &CGF) = 0;
   virtual llvm::Value *EmitTypeid(CodeGenFunction &CGF, QualType SrcRecordTy,
                                   Address ThisPtr,

clang/lib/CodeGen/CGExprCXX.cpp

@@ -2143,40 +2143,9 @@ void CodeGenFunction::EmitCXXDeleteExpr(const CXXDeleteExpr *E) {
   }
 }
 
-static bool isGLValueFromPointerDeref(const Expr *E) {
-  E = E->IgnoreParens();
-
-  if (const auto *CE = dyn_cast<CastExpr>(E)) {
-    if (!CE->getSubExpr()->isGLValue())
-      return false;
-    return isGLValueFromPointerDeref(CE->getSubExpr());
-  }
-
-  if (const auto *OVE = dyn_cast<OpaqueValueExpr>(E))
-    return isGLValueFromPointerDeref(OVE->getSourceExpr());
-
-  if (const auto *BO = dyn_cast<BinaryOperator>(E))
-    if (BO->getOpcode() == BO_Comma)
-      return isGLValueFromPointerDeref(BO->getRHS());
-
-  if (const auto *ACO = dyn_cast<AbstractConditionalOperator>(E))
-    return isGLValueFromPointerDeref(ACO->getTrueExpr()) ||
-           isGLValueFromPointerDeref(ACO->getFalseExpr());
-
-  // C++11 [expr.sub]p1:
-  //   The expression E1[E2] is identical (by definition) to *((E1)+(E2))
-  if (isa<ArraySubscriptExpr>(E))
-    return true;
-
-  if (const auto *UO = dyn_cast<UnaryOperator>(E))
-    if (UO->getOpcode() == UO_Deref)
-      return true;
-
-  return false;
-}
-
 static llvm::Value *EmitTypeidFromVTable(CodeGenFunction &CGF, const Expr *E,
-                                         llvm::Type *StdTypeInfoPtrTy) {
+                                         llvm::Type *StdTypeInfoPtrTy,
+                                         bool HasNullCheck) {
   // Get the vtable pointer.
   Address ThisPtr = CGF.EmitLValue(E).getAddress();
 
@@ -2189,16 +2158,11 @@ static llvm::Value *EmitTypeidFromVTable(CodeGenFunction &CGF, const Expr *E,
   CGF.EmitTypeCheck(CodeGenFunction::TCK_DynamicOperation, E->getExprLoc(),
                     ThisPtr, SrcRecordTy);
 
-  // C++ [expr.typeid]p2:
-  //   If the glvalue expression is obtained by applying the unary * operator to
-  //   a pointer and the pointer is a null pointer value, the typeid expression
-  //   throws the std::bad_typeid exception.
-  //
-  // However, this paragraph's intent is not clear.  We choose a very generous
-  // interpretation which implores us to consider comma operators, conditional
-  // operators, parentheses and other such constructs.
-  if (CGF.CGM.getCXXABI().shouldTypeidBeNullChecked(
-          isGLValueFromPointerDeref(E), SrcRecordTy)) {
+  // Whether we need an explicit null pointer check. For example, with the
+  // Microsoft ABI, if this is a call to __RTtypeid, the null pointer check and
+  // exception throw is inside the __RTtypeid(nullptr) call
+  if (HasNullCheck &&
+      CGF.CGM.getCXXABI().shouldTypeidBeNullChecked(SrcRecordTy)) {
     llvm::BasicBlock *BadTypeidBlock =
         CGF.createBasicBlock("typeid.bad_typeid");
     llvm::BasicBlock *EndBlock = CGF.createBasicBlock("typeid.end");
@@ -2244,7 +2208,8 @@ llvm::Value *CodeGenFunction::EmitCXXTypeidExpr(const CXXTypeidExpr *E) {
   //   type) to which the glvalue refers.
   // If the operand is already most derived object, no need to look up vtable.
   if (E->isPotentiallyEvaluated() && !E->isMostDerived(getContext()))
-    return EmitTypeidFromVTable(*this, E->getExprOperand(), PtrTy);
+    return EmitTypeidFromVTable(*this, E->getExprOperand(), PtrTy,
+                                E->hasNullCheck());
 
   QualType OperandTy = E->getExprOperand()->getType();
   return MaybeASCast(CGM.GetAddrOfRTTIDescriptor(OperandTy));

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -178,7 +178,7 @@ public:
     return CatchTypeInfo{getAddrOfRTTIDescriptor(Ty), 0};
   }
 
-  bool shouldTypeidBeNullChecked(bool IsDeref, QualType SrcRecordTy) override;
+  bool shouldTypeidBeNullChecked(QualType SrcRecordTy) override;
   void EmitBadTypeidCall(CodeGenFunction &CGF) override;
   llvm::Value *EmitTypeid(CodeGenFunction &CGF, QualType SrcRecordTy,
                           Address ThisPtr,
@@ -1419,9 +1419,8 @@ static llvm::FunctionCallee getBadTypeidFn(CodeGenFunction &CGF) {
   return CGF.CGM.CreateRuntimeFunction(FTy, "__cxa_bad_typeid");
 }
 
-bool ItaniumCXXABI::shouldTypeidBeNullChecked(bool IsDeref,
-                                              QualType SrcRecordTy) {
-  return IsDeref;
+bool ItaniumCXXABI::shouldTypeidBeNullChecked(QualType SrcRecordTy) {
+  return true;
 }
 
 void ItaniumCXXABI::EmitBadTypeidCall(CodeGenFunction &CGF) {

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -144,7 +144,7 @@ public:
       return CatchTypeInfo{nullptr, 0x40};
   }
 
-  bool shouldTypeidBeNullChecked(bool IsDeref, QualType SrcRecordTy) override;
+  bool shouldTypeidBeNullChecked(QualType SrcRecordTy) override;
   void EmitBadTypeidCall(CodeGenFunction &CGF) override;
   llvm::Value *EmitTypeid(CodeGenFunction &CGF, QualType SrcRecordTy,
                           Address ThisPtr,
@@ -977,11 +977,9 @@ MicrosoftCXXABI::performBaseAdjustment(CodeGenFunction &CGF, Address Value,
                          PolymorphicBase);
 }
 
-bool MicrosoftCXXABI::shouldTypeidBeNullChecked(bool IsDeref,
-                                                QualType SrcRecordTy) {
+bool MicrosoftCXXABI::shouldTypeidBeNullChecked(QualType SrcRecordTy) {
   const CXXRecordDecl *SrcDecl = SrcRecordTy->getAsCXXRecordDecl();
-  return IsDeref &&
-         !getContext().getASTRecordLayout(SrcDecl).hasExtendableVFPtr();
+  return !getContext().getASTRecordLayout(SrcDecl).hasExtendableVFPtr();
 }
 
 static llvm::CallBase *emitRTtypeidCall(CodeGenFunction &CGF,

clang/lib/Sema/SemaExceptionSpec.cpp

@@ -1114,21 +1114,10 @@ static CanThrowResult canTypeidThrow(Sema &S, const CXXTypeidExpr *DC) {
   if (DC->isTypeOperand())
     return CT_Cannot;
 
-  Expr *Op = DC->getExprOperand();
-  if (Op->isTypeDependent())
+  if (DC->isValueDependent())
     return CT_Dependent;
 
-  const RecordType *RT = Op->getType()->getAs<RecordType>();
-  if (!RT)
-    return CT_Cannot;
-
-  if (!cast<CXXRecordDecl>(RT->getDecl())->isPolymorphic())
-    return CT_Cannot;
-
-  if (Op->Classify(S.Context).isPRValue())
-    return CT_Cannot;
-
-  return CT_Can;
+  return DC->hasNullCheck() ? CT_Can : CT_Cannot;
 }
 
 CanThrowResult Sema::canThrow(const Stmt *S) {
@@ -1157,8 +1146,9 @@ CanThrowResult Sema::canThrow(const Stmt *S) {
   }
 
   case Expr::CXXTypeidExprClass:
-    //   - a potentially evaluated typeid expression applied to a glvalue
-    //     expression whose type is a polymorphic class type
+    //   - a potentially evaluated typeid expression applied to a (possibly
+    //     parenthesized) built-in unary * operator applied to a pointer to a
+    //     polymorphic class type
     return canTypeidThrow(*this, cast<CXXTypeidExpr>(S));
 
     //   - a potentially evaluated call to a function, member function, function

clang/test/CXX/drs/cwg21xx.cpp

@@ -11,6 +11,10 @@
 // cxx98-error@-1 {{variadic macros are a C99 feature}}
 #endif
 
+namespace std {
+struct type_info;
+}
+
 namespace cwg2100 { // cwg2100: 12
   template<const int *P, bool = true> struct X {};
   template<typename T> struct A {
@@ -231,6 +235,15 @@ static_assert(!__is_trivially_assignable(NonConstCopy &&, NonConstCopy &&), "");
 #endif
 } // namespace cwg2171
 
+namespace cwg2191 { // cwg2191: 19
+#if __cplusplus >= 201103L
+struct B { virtual void f() { } };
+struct D : B { } d;
+static_assert(noexcept(typeid(d)), "");
+static_assert(!noexcept(typeid(*static_cast<D*>(nullptr))), "");
+#endif
+} // namespace cwg2191
+
 namespace cwg2180 { // cwg2180: yes
   class A {
     A &operator=(const A &); // #cwg2180-A-copy

clang/test/SemaCXX/warn-unused-value.cpp

@@ -102,6 +102,16 @@ void f() {
   Bad b;
   (void)typeid(b.f()); // expected-warning {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
 
+  extern Bad * pb;
+  // This typeid can throw but that is not a side-effect that we care about
+  // warning for since this is idiomatic code
+  (void)typeid(*pb);
+  (void)sizeof(typeid(*pb));
+  (void)typeid(*++pb); // expected-warning {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
+  (void)sizeof(typeid(*++pb)); // expected-warning {{expression with side effects has no effect in an unevaluated context}}
+  // FIXME: we should not warn about this in an unevaluated context
+  // expected-warning@-2 {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
+
   // A dereference of a volatile pointer is a side effecting operation, however
   // since it is idiomatic code, and the alternatives induce higher maintenance
   // costs, it is allowed.

clang/www/cxx_dr_status.html

@@ -12954,7 +12954,7 @@ and <I>POD class</I></td>
     <td><a href="https://cplusplus.github.io/CWG/issues/2191.html">2191</a></td>
     <td>C++17</td>
     <td>Incorrect result for <TT>noexcept(typeid(v))</TT></td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr class="open" id="2192">
     <td><a href="https://cplusplus.github.io/CWG/issues/2192.html">2192</a></td>