Refactor the way we handle diagnosing unused expression results.

Rather than sprinkle calls to DiagnoseUnusedExprResult() around in places where we want diagnostics, we now diagnose unused expression statements and full expressions in a more generic way when acting on the final expression statement. This results in more appropriate diagnostics for [[nodiscard]] where we were previously lacking them, such as when the body of a for loop is not a compound statement.

This patch fixes PR39837.

llvm-svn: 350404

21 files changed, 196 insertions, 122 deletions

diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index 46e4431913e..438ff0e2ed4 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -360,6 +360,11 @@ class Parser : public CodeCompletionHandler {
   /// just a regular sub-expression.
   SourceLocation ExprStatementTokLoc;
 
+  /// Tests whether an expression value is discarded based on token lookahead.
+  /// It will return true if the lexer is currently processing the })
+  /// terminating a GNU statement expression and false otherwise.
+  bool isExprValueDiscarded();
+
 public:
   Parser(Preprocessor &PP, Sema &Actions, bool SkipFunctionBodies);
   ~Parser() override;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 6d002b37104..92bebcae7c9 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -1365,6 +1365,7 @@ public:
   void PopCompoundScope();
 
   sema::CompoundScopeInfo &getCurCompoundScope() const;
+  bool isCurCompoundStmtAStmtExpr() const;
 
   bool hasAnyUnrecoverableErrorsInThisFunction() const;
 
@@ -3685,16 +3686,17 @@ public:
     return MakeFullExpr(Arg, Arg ? Arg->getExprLoc() : SourceLocation());
   }
   FullExprArg MakeFullExpr(Expr *Arg, SourceLocation CC) {
-    return FullExprArg(ActOnFinishFullExpr(Arg, CC).get());
+    return FullExprArg(
+        ActOnFinishFullExpr(Arg, CC, /*DiscardedValue*/ false).get());
   }
   FullExprArg MakeFullDiscardedValueExpr(Expr *Arg) {
     ExprResult FE =
-      ActOnFinishFullExpr(Arg, Arg ? Arg->getExprLoc() : SourceLocation(),
-                          /*DiscardedValue*/ true);
+        ActOnFinishFullExpr(Arg, Arg ? Arg->getExprLoc() : SourceLocation(),
+                            /*DiscardedValue*/ true);
     return FullExprArg(FE.get());
   }
 
-  StmtResult ActOnExprStmt(ExprResult Arg);
+  StmtResult ActOnExprStmt(ExprResult Arg, bool DiscardedValue = true);
   StmtResult ActOnExprStmtError();
 
   StmtResult ActOnNullStmt(SourceLocation SemiLoc,
@@ -5340,13 +5342,12 @@ public:
   CreateMaterializeTemporaryExpr(QualType T, Expr *Temporary,
                                  bool BoundToLvalueReference);
 
-  ExprResult ActOnFinishFullExpr(Expr *Expr) {
-    return ActOnFinishFullExpr(Expr, Expr ? Expr->getExprLoc()
-                                          : SourceLocation());
+  ExprResult ActOnFinishFullExpr(Expr *Expr, bool DiscardedValue) {
+    return ActOnFinishFullExpr(
+        Expr, Expr ? Expr->getExprLoc() : SourceLocation(), DiscardedValue);
   }
   ExprResult ActOnFinishFullExpr(Expr *Expr, SourceLocation CC,
-                                 bool DiscardedValue = false,
-                                 bool IsConstexpr = false);
+                                 bool DiscardedValue, bool IsConstexpr = false);
   StmtResult ActOnFinishFullStmt(Stmt *Stmt);
 
   // Marks SS invalid if it represents an incomplete type.
diff --git a/clang/lib/Parse/ParseObjc.cpp b/clang/lib/Parse/ParseObjc.cpp
index c8d7bda3d6e..bd55f717939 100644
--- a/clang/lib/Parse/ParseObjc.cpp
+++ b/clang/lib/Parse/ParseObjc.cpp
@@ -2741,7 +2741,7 @@ StmtResult Parser::ParseObjCAtStatement(SourceLocation AtLoc) {
 
   // Otherwise, eat the semicolon.
   ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
-  return Actions.ActOnExprStmt(Res);
+  return Actions.ActOnExprStmt(Res, isExprValueDiscarded());
 }
 
 ExprResult Parser::ParseObjCAtExpression(SourceLocation AtLoc) {
diff --git a/clang/lib/Parse/ParseOpenMP.cpp b/clang/lib/Parse/ParseOpenMP.cpp
index 17c3fa3cf2a..dd2a8aae9f2 100644
--- a/clang/lib/Parse/ParseOpenMP.cpp
+++ b/clang/lib/Parse/ParseOpenMP.cpp
@@ -314,7 +314,7 @@ Parser::ParseOpenMPDeclareReductionDirective(AccessSpecifier AS) {
     Actions.ActOnOpenMPDeclareReductionCombinerStart(getCurScope(), D);
     ExprResult CombinerResult =
         Actions.ActOnFinishFullExpr(ParseAssignmentExpression().get(),
-                                    D->getLocation(), /*DiscardedValue=*/true);
+                                    D->getLocation(), /*DiscardedValue*/ false);
     Actions.ActOnOpenMPDeclareReductionCombinerEnd(D, CombinerResult.get());
 
     if (CombinerResult.isInvalid() && Tok.isNot(tok::r_paren) &&
@@ -356,7 +356,7 @@ Parser::ParseOpenMPDeclareReductionDirective(AccessSpecifier AS) {
           if (Actions.getLangOpts().CPlusPlus) {
             InitializerResult = Actions.ActOnFinishFullExpr(
                 ParseAssignmentExpression().get(), D->getLocation(),
-                /*DiscardedValue=*/true);
+                /*DiscardedValue*/ false);
           } else {
             ConsumeToken();
             ParseOpenMPReductionInitializerForDecl(OmpPrivParm);
@@ -364,7 +364,7 @@ Parser::ParseOpenMPDeclareReductionDirective(AccessSpecifier AS) {
         } else {
           InitializerResult = Actions.ActOnFinishFullExpr(
               ParseAssignmentExpression().get(), D->getLocation(),
-              /*DiscardedValue=*/true);
+              /*DiscardedValue*/ false);
         }
         Actions.ActOnOpenMPDeclareReductionInitializerEnd(
             D, InitializerResult.get(), OmpPrivParm);
@@ -1455,7 +1455,7 @@ ExprResult Parser::ParseOpenMPParensExpr(StringRef ClauseName,
   ExprResult LHS(ParseCastExpression(
       /*isUnaryExpression=*/false, /*isAddressOfOperand=*/false, NotTypeCast));
   ExprResult Val(ParseRHSOfBinaryExpression(LHS, prec::Conditional));
-  Val = Actions.ActOnFinishFullExpr(Val.get(), ELoc);
+  Val = Actions.ActOnFinishFullExpr(Val.get(), ELoc, /*DiscardedValue*/ false);
 
   // Parse ')'.
   RLoc = Tok.getLocation();
@@ -1711,7 +1711,8 @@ OMPClause *Parser::ParseOpenMPSingleExprWithArgClause(OpenMPClauseKind Kind,
     SourceLocation ELoc = Tok.getLocation();
     ExprResult LHS(ParseCastExpression(false, false, NotTypeCast));
     Val = ParseRHSOfBinaryExpression(LHS, prec::Conditional);
-    Val = Actions.ActOnFinishFullExpr(Val.get(), ELoc);
+    Val =
+        Actions.ActOnFinishFullExpr(Val.get(), ELoc, /*DiscardedValue*/ false);
   }
 
   // Parse ')'.
@@ -1996,7 +1997,8 @@ bool Parser::ParseOpenMPVarList(OpenMPDirectiveKind DKind,
     Data.ColonLoc = Tok.getLocation();
     SourceLocation ELoc = ConsumeToken();
     ExprResult Tail = ParseAssignmentExpression();
-    Tail = Actions.ActOnFinishFullExpr(Tail.get(), ELoc);
+    Tail =
+        Actions.ActOnFinishFullExpr(Tail.get(), ELoc, /*DiscardedValue*/ false);
     if (Tail.isUsable())
       Data.TailExpr = Tail.get();
     else
diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index 313793c3e8f..2974e6a245b 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -439,7 +439,7 @@ StmtResult Parser::ParseExprStatement() {
 
   // Otherwise, eat the semicolon.
   ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
-  return Actions.ActOnExprStmt(Expr);
+  return Actions.ActOnExprStmt(Expr, isExprValueDiscarded());
 }
 
 /// ParseSEHTryBlockCommon
@@ -958,6 +958,16 @@ bool Parser::ConsumeNullStmt(StmtVector &Stmts) {
   return true;
 }
 
+bool Parser::isExprValueDiscarded() {
+  if (Actions.isCurCompoundStmtAStmtExpr()) {
+    // Look to see if the next two tokens close the statement expression;
+    // if so, this expression statement is the last statement in a
+    // statment expression.
+    return Tok.isNot(tok::r_brace) || NextToken().isNot(tok::r_paren);
+  }
+  return true;
+}
+
 /// ParseCompoundStatementBody - Parse a sequence of statements and invoke the
 /// ActOnCompoundStmt action.  This expects the '{' to be the current token, and
 /// consume the '}' at the end of the block.  It does not manipulate the scope
@@ -1062,7 +1072,7 @@ StmtResult Parser::ParseCompoundStatementBody(bool isStmtExpr) {
         // Eat the semicolon at the end of stmt and convert the expr into a
         // statement.
         ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
-        R = Actions.ActOnExprStmt(Res);
+        R = Actions.ActOnExprStmt(Res, isExprValueDiscarded());
       }
     }
 
@@ -1698,8 +1708,16 @@ StmtResult Parser::ParseForStatement(SourceLocation *TrailingElseLoc) {
     if (!Value.isInvalid()) {
       if (ForEach)
         FirstPart = Actions.ActOnForEachLValueExpr(Value.get());
-      else
-        FirstPart = Actions.ActOnExprStmt(Value);
+      else {
+        // We already know this is not an init-statement within a for loop, so
+        // if we are parsing a C++11 range-based for loop, we should treat this
+        // expression statement as being a discarded value expression because
+        // we will err below. This way we do not warn on an unused expression
+        // that was an error in the first place, like with: for (expr : expr);
+        bool IsRangeBasedFor =
+            getLangOpts().CPlusPlus11 && !ForEach && Tok.is(tok::colon);
+        FirstPart = Actions.ActOnExprStmt(Value, !IsRangeBasedFor);
+      }
     }
 
     if (Tok.is(tok::semi)) {
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index cc792384709..a84116c1bcc 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -647,7 +647,7 @@ bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
       return StmtError();
     Suspend = BuildResolvedCoawaitExpr(Loc, Suspend.get(),
                                        /*IsImplicit*/ true);
-    Suspend = ActOnFinishFullExpr(Suspend.get());
+    Suspend = ActOnFinishFullExpr(Suspend.get(), /*DiscardedValue*/ false);
     if (Suspend.isInvalid()) {
       Diag(Loc, diag::note_coroutine_promise_suspend_implicitly_required)
           << ((Name == "initial_suspend") ? 0 : 1);
@@ -868,7 +868,7 @@ StmtResult Sema::BuildCoreturnStmt(SourceLocation Loc, Expr *E,
   if (PC.isInvalid())
     return StmtError();
 
-  Expr *PCE = ActOnFinishFullExpr(PC.get()).get();
+  Expr *PCE = ActOnFinishFullExpr(PC.get(), /*DiscardedValue*/ false).get();
 
   Stmt *Res = new (Context) CoreturnStmt(Loc, E, PCE, IsImplicit);
   return Res;
@@ -1237,7 +1237,7 @@ bool CoroutineStmtBuilder::makeNewAndDeleteExpr() {
 
   ExprResult NewExpr =
       S.ActOnCallExpr(S.getCurScope(), NewRef.get(), Loc, NewArgs, Loc);
-  NewExpr = S.ActOnFinishFullExpr(NewExpr.get());
+  NewExpr = S.ActOnFinishFullExpr(NewExpr.get(), /*DiscardedValue*/ false);
   if (NewExpr.isInvalid())
     return false;
 
@@ -1263,7 +1263,8 @@ bool CoroutineStmtBuilder::makeNewAndDeleteExpr() {
 
   ExprResult DeleteExpr =
       S.ActOnCallExpr(S.getCurScope(), DeleteRef.get(), Loc, DeleteArgs, Loc);
-  DeleteExpr = S.ActOnFinishFullExpr(DeleteExpr.get());
+  DeleteExpr =
+      S.ActOnFinishFullExpr(DeleteExpr.get(), /*DiscardedValue*/ false);
   if (DeleteExpr.isInvalid())
     return false;
 
@@ -1348,7 +1349,8 @@ bool CoroutineStmtBuilder::makeOnException() {
 
   ExprResult UnhandledException = buildPromiseCall(S, Fn.CoroutinePromise, Loc,
                                                    "unhandled_exception", None);
-  UnhandledException = S.ActOnFinishFullExpr(UnhandledException.get(), Loc);
+  UnhandledException = S.ActOnFinishFullExpr(UnhandledException.get(), Loc,
+                                             /*DiscardedValue*/ false);
   if (UnhandledException.isInvalid())
     return false;
 
@@ -1401,7 +1403,8 @@ bool CoroutineStmtBuilder::makeGroDeclAndReturnStmt() {
          "get_return_object type must no longer be dependent");
 
   if (FnRetType->isVoidType()) {
-    ExprResult Res = S.ActOnFinishFullExpr(this->ReturnValue, Loc);
+    ExprResult Res =
+        S.ActOnFinishFullExpr(this->ReturnValue, Loc, /*DiscardedValue*/ false);
     if (Res.isInvalid())
       return false;
 
@@ -1433,7 +1436,7 @@ bool CoroutineStmtBuilder::makeGroDeclAndReturnStmt() {
   if (Res.isInvalid())
     return false;
 
-  Res = S.ActOnFinishFullExpr(Res.get());
+  Res = S.ActOnFinishFullExpr(Res.get(), /*DiscardedValue*/ false);
   if (Res.isInvalid())
     return false;
 
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 80cafbedf5e..40b0ed37791 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11204,9 +11204,9 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
   //   struct T { S a, b; } t = { Temp(), Temp() }
   //
   // we should destroy the first Temp before constructing the second.
-  ExprResult Result = ActOnFinishFullExpr(Init, VDecl->getLocation(),
-                                          false,
-                                          VDecl->isConstexpr());
+  ExprResult Result =
+      ActOnFinishFullExpr(Init, VDecl->getLocation(),
+                          /*DiscardedValue*/ false, VDecl->isConstexpr());
   if (Result.isInvalid()) {
     VDecl->setInvalidDecl();
     return;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 8973d632550..2b380bf0dcd 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -1205,7 +1205,7 @@ static bool checkTupleLikeDecomposition(Sema &S,
     E = Seq.Perform(S, Entity, Kind, Init);
     if (E.isInvalid())
       return true;
-    E = S.ActOnFinishFullExpr(E.get(), Loc);
+    E = S.ActOnFinishFullExpr(E.get(), Loc, /*DiscardedValue*/ false);
     if (E.isInvalid())
       return true;
     RefVD->setInit(E.get());
@@ -3682,7 +3682,7 @@ void Sema::ActOnFinishCXXInClassMemberInitializer(Decl *D,
   // C++11 [class.base.init]p7:
   //   The initialization of each base and member constitutes a
   //   full-expression.
-  Init = ActOnFinishFullExpr(Init.get(), InitLoc);
+  Init = ActOnFinishFullExpr(Init.get(), InitLoc, /*DiscardedValue*/ false);
   if (Init.isInvalid()) {
     FD->setInvalidDecl();
     return;
@@ -4040,7 +4040,8 @@ Sema::BuildMemberInitializer(ValueDecl *Member, Expr *Init,
     // C++11 [class.base.init]p7:
     //   The initialization of each base and member constitutes a
     //   full-expression.
-    MemberInit = ActOnFinishFullExpr(MemberInit.get(), InitRange.getBegin());
+    MemberInit = ActOnFinishFullExpr(MemberInit.get(), InitRange.getBegin(),
+                                     /*DiscardedValue*/ false);
     if (MemberInit.isInvalid())
       return true;
 
@@ -4095,8 +4096,8 @@ Sema::BuildDelegatingInitializer(TypeSourceInfo *TInfo, Expr *Init,
   // C++11 [class.base.init]p7:
   //   The initialization of each base and member constitutes a
   //   full-expression.
-  DelegationInit = ActOnFinishFullExpr(DelegationInit.get(),
-                                       InitRange.getBegin());
+  DelegationInit = ActOnFinishFullExpr(
+      DelegationInit.get(), InitRange.getBegin(), /*DiscardedValue*/ false);
   if (DelegationInit.isInvalid())
     return true;
 
@@ -4225,7 +4226,8 @@ Sema::BuildBaseInitializer(QualType BaseType, TypeSourceInfo *BaseTInfo,
   // C++11 [class.base.init]p7:
   //   The initialization of each base and member constitutes a
   //   full-expression.
-  BaseInit = ActOnFinishFullExpr(BaseInit.get(), InitRange.getBegin());
+  BaseInit = ActOnFinishFullExpr(BaseInit.get(), InitRange.getBegin(),
+                                 /*DiscardedValue*/ false);
   if (BaseInit.isInvalid())
     return true;
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index f8b991b7c74..aa4b23b15c5 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -4736,8 +4736,9 @@ bool Sema::CheckCXXDefaultArgExpr(SourceLocation CallLoc, FunctionDecl *FD,
     if (Result.isInvalid())
       return true;
 
-    Result = ActOnFinishFullExpr(Result.getAs<Expr>(),
-                                 Param->getOuterLocStart());
+    Result =
+        ActOnFinishFullExpr(Result.getAs<Expr>(), Param->getOuterLocStart(),
+                            /*DiscardedValue*/ false);
     if (Result.isInvalid())
       return true;
 
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 730c426076f..1c210d332e6 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -7803,6 +7803,8 @@ ExprResult Sema::ActOnFinishFullExpr(Expr *FE, SourceLocation CC,
     FullExpr = IgnoredValueConversions(FullExpr.get());
     if (FullExpr.isInvalid())
       return ExprError();
+
+    DiagnoseUnusedExprResult(FullExpr.get());
   }
 
   FullExpr = CorrectDelayedTyposInExpr(FullExpr.get());
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 6dc93d0761e..c1c4572aa27 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1724,7 +1724,7 @@ ExprResult Sema::BuildBlockForLambdaConversion(SourceLocation CurrentLocation,
                                                  /*NRVO=*/false),
       CurrentLocation, Src);
   if (!Init.isInvalid())
-    Init = ActOnFinishFullExpr(Init.get());
+    Init = ActOnFinishFullExpr(Init.get(), /*DiscardedValue*/ false);
 
   if (Init.isInvalid())
     return ExprError();
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 78bef59ff6f..59c4b49bfb1 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -5320,7 +5320,7 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
         LastIteration.get(), UB.get());
     EUB = SemaRef.BuildBinOp(CurScope, InitLoc, BO_Assign, UB.get(),
                              CondOp.get());
-    EUB = SemaRef.ActOnFinishFullExpr(EUB.get());
+    EUB = SemaRef.ActOnFinishFullExpr(EUB.get(), /*DiscardedValue*/ false);
 
     // If we have a combined directive that combines 'distribute', 'for' or
     // 'simd' we need to be able to access the bounds of the schedule of the
@@ -5349,7 +5349,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
                                      LastIteration.get(), CombUB.get());
       CombEUB = SemaRef.BuildBinOp(CurScope, InitLoc, BO_Assign, CombUB.get(),
                                    CombCondOp.get());
-      CombEUB = SemaRef.ActOnFinishFullExpr(CombEUB.get());
+      CombEUB =
+          SemaRef.ActOnFinishFullExpr(CombEUB.get(), /*DiscardedValue*/ false);
 
       const CapturedDecl *CD = cast<CapturedStmt>(AStmt)->getCapturedDecl();
       // We expect to have at least 2 more parameters than the 'parallel'
@@ -5383,7 +5384,7 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
             ? LB.get()
             : SemaRef.ActOnIntegerConstant(SourceLocation(), 0).get();
     Init = SemaRef.BuildBinOp(CurScope, InitLoc, BO_Assign, IV.get(), RHS);
-    Init = SemaRef.ActOnFinishFullExpr(Init.get());
+    Init = SemaRef.ActOnFinishFullExpr(Init.get(), /*DiscardedValue*/ false);
 
     if (isOpenMPLoopBoundSharingDirective(DKind)) {
       Expr *CombRHS =
@@ -5394,7 +5395,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
               : SemaRef.ActOnIntegerConstant(SourceLocation(), 0).get();
       CombInit =
           SemaRef.BuildBinOp(CurScope, InitLoc, BO_Assign, IV.get(), CombRHS);
-      CombInit = SemaRef.ActOnFinishFullExpr(CombInit.get());
+      CombInit =
+          SemaRef.ActOnFinishFullExpr(CombInit.get(), /*DiscardedValue*/ false);
     }
   }
 
@@ -5426,7 +5428,7 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
   if (!Inc.isUsable())
     return 0;
   Inc = SemaRef.BuildBinOp(CurScope, IncLoc, BO_Assign, IV.get(), Inc.get());
-  Inc = SemaRef.ActOnFinishFullExpr(Inc.get());
+  Inc = SemaRef.ActOnFinishFullExpr(Inc.get(), /*DiscardedValue*/ false);
   if (!Inc.isUsable())
     return 0;
 
@@ -5444,7 +5446,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
     // LB = LB + ST
     NextLB =
         SemaRef.BuildBinOp(CurScope, IncLoc, BO_Assign, LB.get(), NextLB.get());
-    NextLB = SemaRef.ActOnFinishFullExpr(NextLB.get());
+    NextLB =
+        SemaRef.ActOnFinishFullExpr(NextLB.get(), /*DiscardedValue*/ false);
     if (!NextLB.isUsable())
       return 0;
     // UB + ST
@@ -5454,7 +5457,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
     // UB = UB + ST
     NextUB =
         SemaRef.BuildBinOp(CurScope, IncLoc, BO_Assign, UB.get(), NextUB.get());
-    NextUB = SemaRef.ActOnFinishFullExpr(NextUB.get());
+    NextUB =
+        SemaRef.ActOnFinishFullExpr(NextUB.get(), /*DiscardedValue*/ false);
     if (!NextUB.isUsable())
       return 0;
     if (isOpenMPLoopBoundSharingDirective(DKind)) {
@@ -5465,7 +5469,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
       // LB = LB + ST
       CombNextLB = SemaRef.BuildBinOp(CurScope, IncLoc, BO_Assign, CombLB.get(),
                                       CombNextLB.get());
-      CombNextLB = SemaRef.ActOnFinishFullExpr(CombNextLB.get());
+      CombNextLB = SemaRef.ActOnFinishFullExpr(CombNextLB.get(),
+                                               /*DiscardedValue*/ false);
       if (!CombNextLB.isUsable())
         return 0;
       // UB + ST
@@ -5476,7 +5481,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
       // UB = UB + ST
       CombNextUB = SemaRef.BuildBinOp(CurScope, IncLoc, BO_Assign, CombUB.get(),
                                       CombNextUB.get());
-      CombNextUB = SemaRef.ActOnFinishFullExpr(CombNextUB.get());
+      CombNextUB = SemaRef.ActOnFinishFullExpr(CombNextUB.get(),
+                                               /*DiscardedValue*/ false);
       if (!CombNextUB.isUsable())
         return 0;
     }
@@ -5497,7 +5503,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
     assert(DistInc.isUsable() && "distribute inc expr was not built");
     DistInc = SemaRef.BuildBinOp(CurScope, DistIncLoc, BO_Assign, IV.get(),
                                  DistInc.get());
-    DistInc = SemaRef.ActOnFinishFullExpr(DistInc.get());
+    DistInc =
+        SemaRef.ActOnFinishFullExpr(DistInc.get(), /*DiscardedValue*/ false);
     assert(DistInc.isUsable() && "distribute inc expr was not built");
 
     // Build expression: UB = min(UB, prevUB) for #for in composite or combined
@@ -5509,7 +5516,8 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
         DistEUBLoc, DistEUBLoc, IsUBGreater.get(), PrevUB.get(), UB.get());
     PrevEUB = SemaRef.BuildBinOp(CurScope, DistIncLoc, BO_Assign, UB.get(),
                                  CondOp.get());
-    PrevEUB = SemaRef.ActOnFinishFullExpr(PrevEUB.get());
+    PrevEUB =
+        SemaRef.ActOnFinishFullExpr(PrevEUB.get(), /*DiscardedValue*/ false);
 
     // Build IV <= PrevUB to be used in parallel for is in combination with
     // a distribute directive with schedule(static, 1)
@@ -5613,8 +5621,10 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
   Built.IterationVarRef = IV.get();
   Built.LastIteration = LastIteration.get();
   Built.NumIterations = NumIterations.get();
-  Built.CalcLastIteration =
-      SemaRef.ActOnFinishFullExpr(CalcLastIteration.get()).get();
+  Built.CalcLastIteration = SemaRef
+                                .ActOnFinishFullExpr(CalcLastIteration.get(),
+                                                     /*DiscardedValue*/ false)
+                                .get();
   Built.PreCond = PreCond.get();
   Built.PreInits = buildPreInits(C, Captures);
   Built.Cond = Cond.get();
@@ -10267,8 +10277,8 @@ OMPClause *Sema::ActOnOpenMPLastprivateClause(ArrayRef<Expr *> VarList,
                                          PseudoDstExpr, PseudoSrcExpr);
     if (AssignmentOp.isInvalid())
       continue;
-    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc,
-                                       /*DiscardedValue=*/true);
+    AssignmentOp =
+        ActOnFinishFullExpr(AssignmentOp.get(), ELoc, /*DiscardedValue*/ false);
     if (AssignmentOp.isInvalid())
       continue;
 
@@ -11274,7 +11284,8 @@ static bool actOnOMPReductionKindClause(
                            BO_Assign, LHSDRE, ConditionalOp);
         }
         if (ReductionOp.isUsable())
-          ReductionOp = S.ActOnFinishFullExpr(ReductionOp.get());
+          ReductionOp = S.ActOnFinishFullExpr(ReductionOp.get(),
+                                              /*DiscardedValue*/ false);
       }
       if (!ReductionOp.isUsable())
         continue;
@@ -11612,7 +11623,7 @@ OMPClause *Sema::ActOnOpenMPLinearClause(
         buildDeclRefExpr(*this, SaveVar, StepExpr->getType(), StepLoc);
     ExprResult CalcStep =
         BuildBinOp(CurScope, StepLoc, BO_Assign, SaveRef.get(), StepExpr);
-    CalcStep = ActOnFinishFullExpr(CalcStep.get());
+    CalcStep = ActOnFinishFullExpr(CalcStep.get(), /*DiscardedValue*/ false);
 
     // Warn about zero linear step (it would be probably better specified as
     // making corresponding variables 'const').
@@ -11700,7 +11711,7 @@ static bool FinishOpenMPLinearClause(OMPLinearClause &Clause, DeclRefExpr *IV,
     else
       Update = *CurPrivate;
     Update = SemaRef.ActOnFinishFullExpr(Update.get(), DE->getBeginLoc(),
-                                         /*DiscardedValue=*/true);
+                                         /*DiscardedValue*/ false);
 
     // Build final: Var = InitExpr + NumIterations * Step
     ExprResult Final;
@@ -11711,7 +11722,7 @@ static bool FinishOpenMPLinearClause(OMPLinearClause &Clause, DeclRefExpr *IV,
     else
       Final = *CurPrivate;
     Final = SemaRef.ActOnFinishFullExpr(Final.get(), DE->getBeginLoc(),
-                                        /*DiscardedValue=*/true);
+                                        /*DiscardedValue*/ false);
 
     if (!Update.isUsable() || !Final.isUsable()) {
       Updates.push_back(nullptr);
@@ -11879,7 +11890,7 @@ OMPClause *Sema::ActOnOpenMPCopyinClause(ArrayRef<Expr *> VarList,
     if (AssignmentOp.isInvalid())
       continue;
     AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), DE->getExprLoc(),
-                                       /*DiscardedValue=*/true);
+                                       /*DiscardedValue*/ false);
     if (AssignmentOp.isInvalid())
       continue;
 
@@ -11987,8 +11998,8 @@ OMPClause *Sema::ActOnOpenMPCopyprivateClause(ArrayRef<Expr *> VarList,
         DSAStack->getCurScope(), ELoc, BO_Assign, PseudoDstExpr, PseudoSrcExpr);
     if (AssignmentOp.isInvalid())
       continue;
-    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc,
-                                       /*DiscardedValue=*/true);
+    AssignmentOp =
+        ActOnFinishFullExpr(AssignmentOp.get(), ELoc, /*DiscardedValue*/ false);
     if (AssignmentOp.isInvalid())
       continue;
 
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index dacf8d0cf4e..9e30c9a396c 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -42,12 +42,11 @@
 using namespace clang;
 using namespace sema;
 
-StmtResult Sema::ActOnExprStmt(ExprResult FE) {
+StmtResult Sema::ActOnExprStmt(ExprResult FE, bool DiscardedValue) {
   if (FE.isInvalid())
     return StmtError();
 
-  FE = ActOnFinishFullExpr(FE.get(), FE.get()->getExprLoc(),
-                           /*DiscardedValue*/ true);
+  FE = ActOnFinishFullExpr(FE.get(), FE.get()->getExprLoc(), DiscardedValue);
   if (FE.isInvalid())
     return StmtError();
 
@@ -348,6 +347,10 @@ sema::CompoundScopeInfo &Sema::getCurCompoundScope() const {
   return getCurFunction()->CompoundScopes.back();
 }
 
+bool Sema::isCurCompoundStmtAStmtExpr() const {
+  return getCurCompoundScope().IsStmtExpr;
+}
+
 StmtResult Sema::ActOnCompoundStmt(SourceLocation L, SourceLocation R,
                                    ArrayRef<Stmt *> Elts, bool isStmtExpr) {
   const unsigned NumElts = Elts.size();
@@ -370,14 +373,6 @@ StmtResult Sema::ActOnCompoundStmt(SourceLocation L, SourceLocation R,
       Diag(D->getLocation(), diag::ext_mixed_decls_code);
     }
   }
-  // Warn about unused expressions in statements.
-  for (unsigned i = 0; i != NumElts; ++i) {
-    // Ignore statements that are last in a statement expression.
-    if (isStmtExpr && i == NumElts - 1)
-      continue;
-
-    DiagnoseUnusedExprResult(Elts[i]);
-  }
 
   // Check for suspicious empty body (null statement) in `for' and `while'
   // statements.  Don't do anything for template instantiations, this just adds
@@ -469,15 +464,12 @@ Sema::ActOnCaseStmt(SourceLocation CaseLoc, ExprResult LHSVal,
 
 /// ActOnCaseStmtBody - This installs a statement as the body of a case.
 void Sema::ActOnCaseStmtBody(Stmt *S, Stmt *SubStmt) {
-  DiagnoseUnusedExprResult(SubStmt);
   cast<CaseStmt>(S)->setSubStmt(SubStmt);
 }
 
 StmtResult
 Sema::ActOnDefaultStmt(SourceLocation DefaultLoc, SourceLocation ColonLoc,
                        Stmt *SubStmt, Scope *CurScope) {
-  DiagnoseUnusedExprResult(SubStmt);
-
   if (getCurFunction()->SwitchStack.empty()) {
     Diag(DefaultLoc, diag::err_default_not_in_switch);
     return SubStmt;
@@ -571,9 +563,6 @@ StmtResult Sema::BuildIfStmt(SourceLocation IfLoc, bool IsConstexpr,
   if (IsConstexpr || isa<ObjCAvailabilityCheckExpr>(Cond.get().second))
     setFunctionHasBranchProtectedScope();
 
-  DiagnoseUnusedExprResult(thenStmt);
-  DiagnoseUnusedExprResult(elseStmt);
-
   return IfStmt::Create(Context, IfLoc, IsConstexpr, InitStmt, Cond.get().first,
                         Cond.get().second, thenStmt, ElseLoc, elseStmt);
 }
@@ -1301,8 +1290,6 @@ StmtResult Sema::ActOnWhileStmt(SourceLocation WhileLoc, ConditionResult Cond,
       !Diags.isIgnored(diag::warn_comma_operator, CondVal.second->getExprLoc()))
     CommaVisitor(*this).Visit(CondVal.second);
 
-  DiagnoseUnusedExprResult(Body);
-
   if (isa<NullStmt>(Body))
     getCurCompoundScope().setHasEmptyLoopBodies();
 
@@ -1322,7 +1309,7 @@ Sema::ActOnDoStmt(SourceLocation DoLoc, Stmt *Body,
     return StmtError();
   Cond = CondResult.get();
 
-  CondResult = ActOnFinishFullExpr(Cond, DoLoc);
+  CondResult = ActOnFinishFullExpr(Cond, DoLoc, /*DiscardedValue*/ false);
   if (CondResult.isInvalid())
     return StmtError();
   Cond = CondResult.get();
@@ -1332,8 +1319,6 @@ Sema::ActOnDoStmt(SourceLocation DoLoc, Stmt *Body,
       !Diags.isIgnored(diag::warn_comma_operator, Cond->getExprLoc()))
     CommaVisitor(*this).Visit(Cond);
 
-  DiagnoseUnusedExprResult(Body);
-
   return new (Context) DoStmt(Body, Cond, DoLoc, WhileLoc, CondRParen);
 }
 
@@ -1778,11 +1763,6 @@ StmtResult Sema::ActOnForStmt(SourceLocation ForLoc, SourceLocation LParenLoc,
     CommaVisitor(*this).Visit(Second.get().second);
 
   Expr *Third  = third.release().getAs<Expr>();
-
-  DiagnoseUnusedExprResult(First);
-  DiagnoseUnusedExprResult(Third);
-  DiagnoseUnusedExprResult(Body);
-
   if (isa<NullStmt>(Body))
     getCurCompoundScope().setHasEmptyLoopBodies();
 
@@ -1802,7 +1782,7 @@ StmtResult Sema::ActOnForEachLValueExpr(Expr *E) {
   if (result.isInvalid()) return StmtError();
   E = result.get();
 
-  ExprResult FullExpr = ActOnFinishFullExpr(E);
+  ExprResult FullExpr = ActOnFinishFullExpr(E, /*DiscardedValue*/ false);
   if (FullExpr.isInvalid())
     return StmtError();
   return StmtResult(static_cast<Stmt*>(FullExpr.get()));
@@ -1956,7 +1936,8 @@ Sema::ActOnObjCForCollectionStmt(SourceLocation ForLoc,
   if (CollectionExprResult.isInvalid())
     return StmtError();
 
-  CollectionExprResult = ActOnFinishFullExpr(CollectionExprResult.get());
+  CollectionExprResult =
+      ActOnFinishFullExpr(CollectionExprResult.get(), /*DiscardedValue*/ false);
   if (CollectionExprResult.isInvalid())
     return StmtError();
 
@@ -2593,7 +2574,8 @@ StmtResult Sema::BuildCXXForRangeStmt(SourceLocation ForLoc,
     if (!NotEqExpr.isInvalid())
       NotEqExpr = CheckBooleanCondition(ColonLoc, NotEqExpr.get());
     if (!NotEqExpr.isInvalid())
-      NotEqExpr = ActOnFinishFullExpr(NotEqExpr.get());
+      NotEqExpr =
+          ActOnFinishFullExpr(NotEqExpr.get(), /*DiscardedValue*/ false);
     if (NotEqExpr.isInvalid()) {
       Diag(RangeLoc, diag::note_for_range_invalid_iterator)
         << RangeLoc << 0 << BeginRangeRef.get()->getType();
@@ -2616,7 +2598,7 @@ StmtResult Sema::BuildCXXForRangeStmt(SourceLocation ForLoc,
       // co_await during the initial parse.
       IncrExpr = ActOnCoawaitExpr(S, CoawaitLoc, IncrExpr.get());
     if (!IncrExpr.isInvalid())
-      IncrExpr = ActOnFinishFullExpr(IncrExpr.get());
+      IncrExpr = ActOnFinishFullExpr(IncrExpr.get(), /*DiscardedValue*/ false);
     if (IncrExpr.isInvalid()) {
       Diag(RangeLoc, diag::note_for_range_invalid_iterator)
         << RangeLoc << 2 << BeginRangeRef.get()->getType() ;
@@ -2871,7 +2853,7 @@ Sema::ActOnIndirectGotoStmt(SourceLocation GotoLoc, SourceLocation StarLoc,
       return StmtError();
   }
 
-  ExprResult ExprRes = ActOnFinishFullExpr(E);
+  ExprResult ExprRes = ActOnFinishFullExpr(E, /*DiscardedValue*/ false);
   if (ExprRes.isInvalid())
     return StmtError();
   E = ExprRes.get();
@@ -3221,7 +3203,8 @@ Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
           ExpressionEvaluationContext::DiscardedStatement &&
       (HasDeducedReturnType || CurCap->HasImplicitReturnType)) {
     if (RetValExp) {
-      ExprResult ER = ActOnFinishFullExpr(RetValExp, ReturnLoc);
+      ExprResult ER =
+          ActOnFinishFullExpr(RetValExp, ReturnLoc, /*DiscardedValue*/ false);
       if (ER.isInvalid())
         return StmtError();
       RetValExp = ER.get();
@@ -3348,7 +3331,8 @@ Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
   }
 
   if (RetValExp) {
-    ExprResult ER = ActOnFinishFullExpr(RetValExp, ReturnLoc);
+    ExprResult ER =
+        ActOnFinishFullExpr(RetValExp, ReturnLoc, /*DiscardedValue*/ false);
     if (ER.isInvalid())
       return StmtError();
     RetValExp = ER.get();
@@ -3578,7 +3562,8 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
           ExpressionEvaluationContext::DiscardedStatement &&
       FnRetType->getContainedAutoType()) {
     if (RetValExp) {
-      ExprResult ER = ActOnFinishFullExpr(RetValExp, ReturnLoc);
+      ExprResult ER =
+          ActOnFinishFullExpr(RetValExp, ReturnLoc, /*DiscardedValue*/ false);
       if (ER.isInvalid())
         return StmtError();
       RetValExp = ER.get();
@@ -3672,7 +3657,8 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
       }
 
       if (RetValExp) {
-        ExprResult ER = ActOnFinishFullExpr(RetValExp, ReturnLoc);
+        ExprResult ER =
+            ActOnFinishFullExpr(RetValExp, ReturnLoc, /*DiscardedValue*/ false);
         if (ER.isInvalid())
           return StmtError();
         RetValExp = ER.get();
@@ -3751,7 +3737,8 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
     }
 
     if (RetValExp) {
-      ExprResult ER = ActOnFinishFullExpr(RetValExp, ReturnLoc);
+      ExprResult ER =
+          ActOnFinishFullExpr(RetValExp, ReturnLoc, /*DiscardedValue*/ false);
       if (ER.isInvalid())
         return StmtError();
       RetValExp = ER.get();
@@ -3804,7 +3791,7 @@ StmtResult Sema::BuildObjCAtThrowStmt(SourceLocation AtLoc, Expr *Throw) {
     if (Result.isInvalid())
       return StmtError();
 
-    Result = ActOnFinishFullExpr(Result.get());
+    Result = ActOnFinishFullExpr(Result.get(), /*DiscardedValue*/ false);
     if (Result.isInvalid())
       return StmtError();
     Throw = Result.get();
@@ -3876,7 +3863,7 @@ Sema::ActOnObjCAtSynchronizedOperand(SourceLocation atLoc, Expr *operand) {
   }
 
   // The operand to @synchronized is a full-expression.
-  return ActOnFinishFullExpr(operand);
+  return ActOnFinishFullExpr(operand, /*DiscardedValue*/ false);
 }
 
 StmtResult
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9de4e8d654f..df14768cbe8 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -328,7 +328,7 @@ public:
   /// other mechanism.
   ///
   /// \returns the transformed statement.
-  StmtResult TransformStmt(Stmt *S);
+  StmtResult TransformStmt(Stmt *S, bool DiscardedValue = false);
 
   /// Transform the given statement.
   ///
@@ -3269,8 +3269,8 @@ private:
                                       bool DeducibleTSTContext);
 };
 
-template<typename Derived>
-StmtResult TreeTransform<Derived>::TransformStmt(Stmt *S) {
+template <typename Derived>
+StmtResult TreeTransform<Derived>::TransformStmt(Stmt *S, bool DiscardedValue) {
   if (!S)
     return S;
 
@@ -3294,7 +3294,7 @@ StmtResult TreeTransform<Derived>::TransformStmt(Stmt *S) {
       if (E.isInvalid())
         return StmtError();
 
-      return getSema().ActOnExprStmt(E);
+      return getSema().ActOnExprStmt(E, DiscardedValue);
     }
   }
 
@@ -4715,7 +4715,8 @@ TreeTransform<Derived>::TransformVariableArrayType(TypeLocBuilder &TLB,
   }
   if (SizeResult.isInvalid())
     return QualType();
-  SizeResult = SemaRef.ActOnFinishFullExpr(SizeResult.get());
+  SizeResult =
+      SemaRef.ActOnFinishFullExpr(SizeResult.get(), /*DiscardedValue*/ false);
   if (SizeResult.isInvalid())
     return QualType();
 
@@ -6520,7 +6521,9 @@ TreeTransform<Derived>::TransformCompoundStmt(CompoundStmt *S,
   bool SubStmtChanged = false;
   SmallVector<Stmt*, 8> Statements;
   for (auto *B : S->body()) {
-    StmtResult Result = getDerived().TransformStmt(B);
+    StmtResult Result =
+        getDerived().TransformStmt(B, !IsStmtExpr || B != S->body_back());
+
     if (Result.isInvalid()) {
       // Immediately fail if this was a DeclStmt, since it's very
       // likely that this will cause problems for future statements.
diff --git a/clang/test/CXX/stmt.stmt/stmt.select/p3.cpp b/clang/test/CXX/stmt.stmt/stmt.select/p3.cpp
index 7a6a408ec95..4804cc559d0 100644
--- a/clang/test/CXX/stmt.stmt/stmt.select/p3.cpp
+++ b/clang/test/CXX/stmt.stmt/stmt.select/p3.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fsyntax-only -std=c++1z -Wc++14-compat -verify %s -DCPP17
+// RUN: %clang_cc1 -fsyntax-only -Wno-unused-value -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wno-unused-value -std=c++1z -Wc++14-compat -verify %s -DCPP17
 
 int f();
 
@@ -71,7 +71,6 @@ void whileInitStatement() {
 // last loop above. It would be nice to remove this.
 void whileInitStatement2() {
   while (; false) {} // expected-error {{expected expression}}
-  // expected-warning@-1 {{expression result unused}}
-  // expected-error@-2 {{expected ';' after expression}}
-  // expected-error@-3 {{expected expression}}
+  // expected-error@-1 {{expected ';' after expression}}
+  // expected-error@-2 {{expected expression}}
 }
diff --git a/clang/test/CodeCompletion/pragma-macro-token-caching.c b/clang/test/CodeCompletion/pragma-macro-token-caching.c
index 432706e85ce..59b6621b56a 100644
--- a/clang/test/CodeCompletion/pragma-macro-token-caching.c
+++ b/clang/test/CodeCompletion/pragma-macro-token-caching.c
@@ -12,7 +12,7 @@ void completeParam(int param) {
 
 void completeParamPragmaError(int param) {
     Outer(__extension__({ _Pragma(2) })); // expected-error {{_Pragma takes a parenthesized string literal}}
-    param;
+    param; // expected-warning {{expression result unused}}
 }
 
 // RUN: %clang_cc1 -fsyntax-only -verify -code-completion-at=%s:16:1 %s | FileCheck %s
diff --git a/clang/test/Parser/cxx1z-init-statement.cpp b/clang/test/Parser/cxx1z-init-statement.cpp
index 3d119ef8e70..ade60dc762d 100644
--- a/clang/test/Parser/cxx1z-init-statement.cpp
+++ b/clang/test/Parser/cxx1z-init-statement.cpp
@@ -13,9 +13,9 @@ int f() {
   if (T(n) = 0; n) {}
 
   // init-statement expressions
-  if (T{f()}; f()) {}
-  if (T{f()}, g, h; f()) {} // expected-warning 2{{unused}}
-  if (T(f()), g, h + 1; f()) {} // expected-warning 2{{unused}}
+  if (T{f()}; f()) {} // expected-warning {{expression result unused}}
+  if (T{f()}, g, h; f()) {} // expected-warning 2{{unused}} expected-warning {{expression result unused}}
+  if (T(f()), g, h + 1; f()) {} // expected-warning 2{{unused}} expected-warning {{expression result unused}}
 
   // condition declarations
   if (T(n){g}) {}
@@ -35,7 +35,7 @@ int f() {
 
   // Likewise for 'switch'
   switch (int n; n) {}
-  switch (g; int g = 5) {}
+  switch (g; int g = 5) {} // expected-warning {{expression result unused}}
 
   if (int a, b; int c = a) { // expected-note 6{{previous}}
     int a; // expected-error {{redefinition}}
diff --git a/clang/test/Parser/switch-recovery.cpp b/clang/test/Parser/switch-recovery.cpp
index a3a0178cd10..eacd017ab23 100644
--- a/clang/test/Parser/switch-recovery.cpp
+++ b/clang/test/Parser/switch-recovery.cpp
@@ -105,7 +105,7 @@ void test9(int x) { // expected-note {{'x' declared here}}
               expected-error {{expected expression}}
     8:: x; // expected-error {{expected ';' after expression}} \
               expected-error {{no member named 'x' in the global namespace; did you mean simply 'x'?}} \
-              expected-warning 2 {{expression result unused}}
+              expected-warning {{expression result unused}}
     9:: :y; // expected-error {{expected ';' after expression}} \
                expected-error {{expected unqualified-id}} \
                expected-warning {{expression result unused}}
diff --git a/clang/test/SemaCXX/cxx1z-init-statement.cpp b/clang/test/SemaCXX/cxx1z-init-statement.cpp
index d37acd08ce7..eea2589ab7c 100644
--- a/clang/test/SemaCXX/cxx1z-init-statement.cpp
+++ b/clang/test/SemaCXX/cxx1z-init-statement.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++1z -verify %s
-// RUN: %clang_cc1 -std=c++17 -verify %s
+// RUN: %clang_cc1 -std=c++1z -Wno-unused-value -verify %s
+// RUN: %clang_cc1 -std=c++17 -Wno-unused-value -verify %s
 
 void testIf() {
   int x = 0;
@@ -12,7 +12,7 @@ void testIf() {
     int x = 0; // expected-error {{redefinition of 'x'}}
 
   if (x; int a = 0) ++a;
-  if (x, +x; int a = 0) // expected-note 2 {{previous definition is here}} expected-warning {{unused}}
+  if (x, +x; int a = 0) // expected-note 2 {{previous definition is here}}
     int a = 0; // expected-error {{redefinition of 'a'}}
   else
     int a = 0; // expected-error {{redefinition of 'a'}}
@@ -48,7 +48,7 @@ void testSwitch() {
       ++a;
   }
 
-  switch (x, +x; int a = 0) { // expected-note {{previous definition is here}} expected-warning {{unused}}
+  switch (x, +x; int a = 0) { // expected-note {{previous definition is here}}
     case 0:
       int a = 0; // expected-error {{redefinition of 'a'}} // expected-note {{previous definition is here}}
     case 1:
diff --git a/clang/test/SemaCXX/for-range-examples.cpp b/clang/test/SemaCXX/for-range-examples.cpp
index 477789b56c1..5424b7a8ee7 100644
--- a/clang/test/SemaCXX/for-range-examples.cpp
+++ b/clang/test/SemaCXX/for-range-examples.cpp
@@ -244,7 +244,7 @@ void foo ()
 { 
   int b = 1, a[b];
   a[0] = 0;
-  [&] { for (int c : a) 0; } ();
+  [&] { for (int c : a) 0; } (); // expected-warning {{expression result unused}}
 }
 
 
diff --git a/clang/test/SemaCXX/warn-unused-result.cpp b/clang/test/SemaCXX/warn-unused-result.cpp
index 88f5ab1e85c..f1de4618a74 100644
--- a/clang/test/SemaCXX/warn-unused-result.cpp
+++ b/clang/test/SemaCXX/warn-unused-result.cpp
@@ -33,6 +33,36 @@ void test() {
   const S &s4 = g1();
 }
 
+void testSubstmts(int i) {
+  switch (i) {
+  case 0:
+    f(); // expected-warning {{ignoring return value}}
+  default:
+    f(); // expected-warning {{ignoring return value}}
+  }
+
+  if (i)
+    f(); // expected-warning {{ignoring return value}}
+  else
+    f(); // expected-warning {{ignoring return value}}
+
+  while (i)
+    f(); // expected-warning {{ignoring return value}}
+
+  do
+    f(); // expected-warning {{ignoring return value}}
+  while (i);
+
+  for (f(); // expected-warning {{ignoring return value}}
+       ;
+       f() // expected-warning {{ignoring return value}}
+      )
+    f(); // expected-warning {{ignoring return value}}
+
+  f(),  // expected-warning {{ignoring return value}}
+  (void)f();
+}
+
 struct X {
  int foo() __attribute__((warn_unused_result));
 };
@@ -206,3 +236,13 @@ void f() {
   (void)++p;
 }
 } // namespace
+
+namespace PR39837 {
+[[clang::warn_unused_result]] int f(int);
+
+void g() {
+  int a[2];
+  for (int b : a)
+    f(b); // expected-warning {{ignoring return value}}
+}
+} // namespace PR39837