[Clang] Implement CWG2813: Class member access with prvalues

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9182,9 +9182,6 @@ def warn_unused_constructor : Warning<
 def warn_unused_constructor_msg : Warning<
   "ignoring temporary created by a constructor declared with %0 attribute: %1">,
   InGroup<UnusedValue>;
-def warn_discarded_class_member_access : Warning<
-  "left operand of dot in this class member access is discarded and has no effect">,
-  InGroup<UnusedValue>;
 def warn_side_effects_unevaluated_context : Warning<
   "expression with side effects has no effect in an unevaluated context">,
   InGroup<UnevaluatedExpression>;

clang/include/clang/Sema/Sema.h

@@ -8502,6 +8502,11 @@ class Sema final : public SemaBase {
                            SourceLocation EndLoc);
   void ActOnForEachDeclStmt(DeclGroupPtrTy Decl);
 
+  /// DiagnoseDiscardedNodiscard - Given an expression that is semantically
+  /// a discarded-value expression, diagnose if any [[nodiscard]] value
+  /// has been discarded
+  void DiagnoseDiscardedNodiscard(const Expr *E);
+
   /// DiagnoseUnusedExprResult - If the statement passed in is an expression
   /// whose result is unused, warn.
   void DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID);

clang/lib/AST/Expr.cpp

@@ -2962,6 +2962,9 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
   case ExprWithCleanupsClass:
     return cast<ExprWithCleanups>(this)->getSubExpr()
                ->isUnusedResultAWarning(WarnE, Loc, R1, R2, Ctx);
+  case OpaqueValueExprClass:
+    return cast<OpaqueValueExpr>(this)->getSourceExpr()->isUnusedResultAWarning(
+        WarnE, Loc, R1, R2, Ctx);
   }
 }
 

clang/lib/Sema/SemaExprMember.cpp

@@ -1136,7 +1136,7 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
   //   an enumerator, the first expression is a discarded-value expression; if
   //   the id-expression names a non-static data member, the first expression
   //   shall be a glvalue.
-  auto MakeDiscardedValue = [&BaseExpr, IsArrow, this] {
+  auto MakeDiscardedValue = [&] {
     assert(getLangOpts().CPlusPlus &&
            "Static member / member enumerator outside of C++");
     if (IsArrow)
@@ -1145,11 +1145,10 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
     if (Converted.isInvalid())
       return true;
     BaseExpr = Converted.get();
-    DiagnoseUnusedExprResult(BaseExpr,
-                             diag::warn_discarded_class_member_access);
+    DiagnoseDiscardedNodiscard(BaseExpr);
     return false;
   };
-  auto MakeGLValue = [&BaseExpr, IsArrow, this] {
+  auto MakeGLValue = [&] {
     if (IsArrow || !BaseExpr->isPRValue())
       return false;
     ExprResult Converted = TemporaryMaterializationConversion(BaseExpr);
@@ -1171,10 +1170,11 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
   }
 
   if (MSPropertyDecl *PD = dyn_cast<MSPropertyDecl>(MemberDecl)) {
-    // Properties treated as non-static data members for the purpose of
-    // temporary materialization
-    if (MakeGLValue())
-      return ExprError();
+    // No temporaries are materialized for property references yet.
+    // They might be materialized when this is transformed into a member call.
+    // Note that this is slightly different behaviour from MSVC which doesn't
+    // implement CWG2813 yet: MSVC might materialize an extra temporary if the
+    // getter or setter function is an explicit object member function.
     return BuildMSPropertyRefExpr(*this, BaseExpr, IsArrow, SS, PD,
                                   MemberNameInfo);
   }

clang/lib/Sema/SemaStmt.cpp

@@ -222,18 +222,13 @@ static bool DiagnoseNoDiscard(Sema &S, const WarnUnusedResultAttr *A,
   return S.Diag(Loc, diag::warn_unused_result_msg) << A << Msg << R1 << R2;
 }
 
-void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
-  const unsigned OrigDiagID = DiagID;
-  if (const LabelStmt *Label = dyn_cast_or_null<LabelStmt>(S))
-    return DiagnoseUnusedExprResult(Label->getSubStmt(), DiagID);
-
-  const Expr *E = dyn_cast_or_null<Expr>(S);
-  if (!E)
-    return;
+namespace {
+void DiagnoseUnused(Sema &S, const Expr *E, std::optional<unsigned> DiagID) {
+  bool NoDiscardOnly = !DiagID.has_value();
 
   // If we are in an unevaluated expression context, then there can be no unused
   // results because the results aren't expected to be used in the first place.
-  if (isUnevaluatedContext())
+  if (S.isUnevaluatedContext())
     return;
 
   SourceLocation ExprLoc = E->IgnoreParenImpCasts()->getExprLoc();
@@ -242,30 +237,31 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   // expression is a call to a function with the warn_unused_result attribute,
   // we warn no matter the location. Because of the order in which the various
   // checks need to happen, we factor out the macro-related test here.
-  bool ShouldSuppress =
-      SourceMgr.isMacroBodyExpansion(ExprLoc) ||
-      SourceMgr.isInSystemMacro(ExprLoc);
+  bool ShouldSuppress = S.SourceMgr.isMacroBodyExpansion(ExprLoc) ||
+                        S.SourceMgr.isInSystemMacro(ExprLoc);
 
   const Expr *WarnExpr;
   SourceLocation Loc;
   SourceRange R1, R2;
-  if (!E->isUnusedResultAWarning(WarnExpr, Loc, R1, R2, Context))
+  if (!E->isUnusedResultAWarning(WarnExpr, Loc, R1, R2, S.Context))
     return;
 
-  // If this is a GNU statement expression expanded from a macro, it is probably
-  // unused because it is a function-like macro that can be used as either an
-  // expression or statement.  Don't warn, because it is almost certainly a
-  // false positive.
-  if (isa<StmtExpr>(E) && Loc.isMacroID())
-    return;
-
-  // Check if this is the UNREFERENCED_PARAMETER from the Microsoft headers.
-  // That macro is frequently used to suppress "unused parameter" warnings,
-  // but its implementation makes clang's -Wunused-value fire.  Prevent this.
-  if (isa<ParenExpr>(E->IgnoreImpCasts()) && Loc.isMacroID()) {
-    SourceLocation SpellLoc = Loc;
-    if (findMacroSpelling(SpellLoc, "UNREFERENCED_PARAMETER"))
+  if (!NoDiscardOnly) {
+    // If this is a GNU statement expression expanded from a macro, it is
+    // probably unused because it is a function-like macro that can be used as
+    // either an expression or statement. Don't warn, because it is almost
+    // certainly a false positive.
+    if (isa<StmtExpr>(E) && Loc.isMacroID())
       return;
+
+    // Check if this is the UNREFERENCED_PARAMETER from the Microsoft headers.
+    // That macro is frequently used to suppress "unused parameter" warnings,
+    // but its implementation makes clang's -Wunused-value fire. Prevent this.
+    if (isa<ParenExpr>(E->IgnoreImpCasts()) && Loc.isMacroID()) {
+      SourceLocation SpellLoc = Loc;
+      if (S.findMacroSpelling(SpellLoc, "UNREFERENCED_PARAMETER"))
+        return;
+    }
   }
 
   // Okay, we have an unused result.  Depending on what the base expression is,
@@ -276,7 +272,7 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   if (const CXXBindTemporaryExpr *TempExpr = dyn_cast<CXXBindTemporaryExpr>(E))
     E = TempExpr->getSubExpr();
 
-  if (DiagnoseUnusedComparison(*this, E))
+  if (DiagnoseUnusedComparison(S, E))
     return;
 
   E = WarnExpr;
@@ -289,8 +285,9 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
     if (E->getType()->isVoidType())
       return;
 
-    if (DiagnoseNoDiscard(*this, cast_or_null<WarnUnusedResultAttr>(
-                                     CE->getUnusedResultAttr(Context)),
+    if (DiagnoseNoDiscard(S,
+                          cast_if_present<WarnUnusedResultAttr>(
+                              CE->getUnusedResultAttr(S.Context)),
                           Loc, R1, R2, /*isCtor=*/false))
       return;
 
@@ -302,50 +299,50 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
       if (ShouldSuppress)
         return;
       if (FD->hasAttr<PureAttr>()) {
-        Diag(Loc, diag::warn_unused_call) << R1 << R2 << "pure";
+        S.Diag(Loc, diag::warn_unused_call) << R1 << R2 << "pure";
         return;
       }
       if (FD->hasAttr<ConstAttr>()) {
-        Diag(Loc, diag::warn_unused_call) << R1 << R2 << "const";
+        S.Diag(Loc, diag::warn_unused_call) << R1 << R2 << "const";
         return;
       }
     }
   } else if (const auto *CE = dyn_cast<CXXConstructExpr>(E)) {
     if (const CXXConstructorDecl *Ctor = CE->getConstructor()) {
       const auto *A = Ctor->getAttr<WarnUnusedResultAttr>();
       A = A ? A : Ctor->getParent()->getAttr<WarnUnusedResultAttr>();
-      if (DiagnoseNoDiscard(*this, A, Loc, R1, R2, /*isCtor=*/true))
+      if (DiagnoseNoDiscard(S, A, Loc, R1, R2, /*isCtor=*/true))
         return;
     }
   } else if (const auto *ILE = dyn_cast<InitListExpr>(E)) {
     if (const TagDecl *TD = ILE->getType()->getAsTagDecl()) {
 
-      if (DiagnoseNoDiscard(*this, TD->getAttr<WarnUnusedResultAttr>(), Loc, R1,
-                            R2, /*isCtor=*/false))
+      if (DiagnoseNoDiscard(S, TD->getAttr<WarnUnusedResultAttr>(), Loc, R1, R2,
+                            /*isCtor=*/false))
         return;
     }
   } else if (ShouldSuppress)
     return;
 
   E = WarnExpr;
   if (const ObjCMessageExpr *ME = dyn_cast<ObjCMessageExpr>(E)) {
-    if (getLangOpts().ObjCAutoRefCount && ME->isDelegateInitCall()) {
-      Diag(Loc, diag::err_arc_unused_init_message) << R1;
+    if (S.getLangOpts().ObjCAutoRefCount && ME->isDelegateInitCall()) {
+      S.Diag(Loc, diag::err_arc_unused_init_message) << R1;
       return;
     }
     const ObjCMethodDecl *MD = ME->getMethodDecl();
     if (MD) {
-      if (DiagnoseNoDiscard(*this, MD->getAttr<WarnUnusedResultAttr>(), Loc, R1,
-                            R2, /*isCtor=*/false))
+      if (DiagnoseNoDiscard(S, MD->getAttr<WarnUnusedResultAttr>(), Loc, R1, R2,
+                            /*isCtor=*/false))
         return;
     }
   } else if (const PseudoObjectExpr *POE = dyn_cast<PseudoObjectExpr>(E)) {
     const Expr *Source = POE->getSyntacticForm();
     // Handle the actually selected call of an OpenMP specialized call.
-    if (LangOpts.OpenMP && isa<CallExpr>(Source) &&
+    if (S.LangOpts.OpenMP && isa<CallExpr>(Source) &&
         POE->getNumSemanticExprs() == 1 &&
         isa<CallExpr>(POE->getSemanticExpr(0)))
-      return DiagnoseUnusedExprResult(POE->getSemanticExpr(0), DiagID);
+      return DiagnoseUnused(S, POE->getSemanticExpr(0), DiagID);
     if (isa<ObjCSubscriptRefExpr>(Source))
       DiagID = diag::warn_unused_container_subscript_expr;
     else if (isa<ObjCPropertyRefExpr>(Source))
@@ -362,17 +359,21 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
         if (!RD->getAttr<WarnUnusedAttr>())
           return;
   }
+
+  if (NoDiscardOnly)
+    return;
+
   // Diagnose "(void*) blah" as a typo for "(void) blah".
-  else if (const CStyleCastExpr *CE = dyn_cast<CStyleCastExpr>(E)) {
+  if (const CStyleCastExpr *CE = dyn_cast<CStyleCastExpr>(E)) {
     TypeSourceInfo *TI = CE->getTypeInfoAsWritten();
     QualType T = TI->getType();
 
     // We really do want to use the non-canonical type here.
-    if (T == Context.VoidPtrTy) {
+    if (T == S.Context.VoidPtrTy) {
       PointerTypeLoc TL = TI->getTypeLoc().castAs<PointerTypeLoc>();
 
-      Diag(Loc, diag::warn_unused_voidptr)
-        << FixItHint::CreateRemoval(TL.getStarLoc());
+      S.Diag(Loc, diag::warn_unused_voidptr)
+          << FixItHint::CreateRemoval(TL.getStarLoc());
       return;
     }
   }
@@ -381,23 +382,34 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   // isn't an array.
   if (E->isGLValue() && E->getType().isVolatileQualified() &&
       !E->getType()->isArrayType()) {
-    Diag(Loc, diag::warn_unused_volatile) << R1 << R2;
+    S.Diag(Loc, diag::warn_unused_volatile) << R1 << R2;
     return;
   }
 
   // Do not diagnose use of a comma operator in a SFINAE context because the
   // type of the left operand could be used for SFINAE, so technically it is
   // *used*.
-  if (DiagID == diag::warn_unused_comma_left_operand && isSFINAEContext())
+  if (DiagID == diag::warn_unused_comma_left_operand && S.isSFINAEContext())
     return;
 
-  // Don't diagnose discarded left of dot in static class member access
-  // because its type is "used" to determine the class to access
-  if (OrigDiagID == diag::warn_discarded_class_member_access)
+  S.DiagIfReachable(Loc, llvm::ArrayRef<const Stmt *>(E),
+                    S.PDiag(*DiagID) << R1 << R2);
+}
+} // namespace
+
+void Sema::DiagnoseDiscardedNodiscard(const Expr *E) {
+  DiagnoseUnused(*this, E, std::nullopt);
+}
+
+void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
+  if (const LabelStmt *Label = dyn_cast_if_present<LabelStmt>(S))
+    S = Label->getSubStmt();
+
+  const Expr *E = dyn_cast_if_present<Expr>(S);
+  if (!E)
     return;
 
-  DiagIfReachable(Loc, S ? llvm::ArrayRef(S) : std::nullopt,
-                  PDiag(DiagID) << R1 << R2);
+  DiagnoseUnused(*this, E, DiagID);
 }
 
 void Sema::ActOnStartOfCompoundStmt(bool IsStmtExpr) {