Index: include/clang/Parse/Parser.h
===================================================================
--- include/clang/Parse/Parser.h
+++ include/clang/Parse/Parser.h
@@ -359,6 +359,8 @@
   /// just a regular sub-expression.
   SourceLocation ExprStatementTokLoc;
 
+  bool warnOnDiscardedValueExpr();
+
 public:
   Parser(Preprocessor &PP, Sema &Actions, bool SkipFunctionBodies);
   ~Parser() override;
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -3692,7 +3692,7 @@
     return FullExprArg(FE.get());
   }
 
-  StmtResult ActOnExprStmt(ExprResult Arg);
+  StmtResult ActOnExprStmt(ExprResult Arg, bool WarnOnDiscardedValue = true);
   StmtResult ActOnExprStmtError();
 
   StmtResult ActOnNullStmt(SourceLocation SemiLoc,
@@ -5334,7 +5334,7 @@
                                           : SourceLocation());
   }
   ExprResult ActOnFinishFullExpr(Expr *Expr, SourceLocation CC,
-                                 bool DiscardedValue = false,
+                                 bool WarnOnDiscardedValue = false,
                                  bool IsConstexpr = false);
   StmtResult ActOnFinishFullStmt(Stmt *Stmt);
 
Index: lib/Parse/ParseObjc.cpp
===================================================================
--- lib/Parse/ParseObjc.cpp
+++ lib/Parse/ParseObjc.cpp
@@ -2741,7 +2741,7 @@
 
   // Otherwise, eat the semicolon.
   ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
-  return Actions.ActOnExprStmt(Res);
+  return Actions.ActOnExprStmt(Res, warnOnDiscardedValueExpr());
 }
 
 ExprResult Parser::ParseObjCAtExpression(SourceLocation AtLoc) {
Index: lib/Parse/ParseOpenMP.cpp
===================================================================
--- lib/Parse/ParseOpenMP.cpp
+++ lib/Parse/ParseOpenMP.cpp
@@ -312,9 +312,8 @@
                                     Scope::OpenMPDirectiveScope);
     // Parse <combiner> expression.
     Actions.ActOnOpenMPDeclareReductionCombinerStart(getCurScope(), D);
-    ExprResult CombinerResult =
-        Actions.ActOnFinishFullExpr(ParseAssignmentExpression().get(),
-                                    D->getLocation(), /*DiscardedValue=*/true);
+    ExprResult CombinerResult = Actions.ActOnFinishFullExpr(
+        ParseAssignmentExpression().get(), D->getLocation());
     Actions.ActOnOpenMPDeclareReductionCombinerEnd(D, CombinerResult.get());
 
     if (CombinerResult.isInvalid() && Tok.isNot(tok::r_paren) &&
@@ -355,16 +354,14 @@
             Tok.getIdentifierInfo()->isStr("omp_priv")) {
           if (Actions.getLangOpts().CPlusPlus) {
             InitializerResult = Actions.ActOnFinishFullExpr(
-                ParseAssignmentExpression().get(), D->getLocation(),
-                /*DiscardedValue=*/true);
+                ParseAssignmentExpression().get(), D->getLocation());
           } else {
             ConsumeToken();
             ParseOpenMPReductionInitializerForDecl(OmpPrivParm);
           }
         } else {
           InitializerResult = Actions.ActOnFinishFullExpr(
-              ParseAssignmentExpression().get(), D->getLocation(),
-              /*DiscardedValue=*/true);
+              ParseAssignmentExpression().get(), D->getLocation());
         }
         Actions.ActOnOpenMPDeclareReductionInitializerEnd(
             D, InitializerResult.get(), OmpPrivParm);
Index: lib/Parse/ParseStmt.cpp
===================================================================
--- lib/Parse/ParseStmt.cpp
+++ lib/Parse/ParseStmt.cpp
@@ -20,6 +20,7 @@
 #include "clang/Parse/RAIIObjectsForParser.h"
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/Scope.h"
+#include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/TypoCorrection.h"
 using namespace clang;
 
@@ -439,7 +440,7 @@
 
   // Otherwise, eat the semicolon.
   ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
-  return Actions.ActOnExprStmt(Expr);
+  return Actions.ActOnExprStmt(Expr, warnOnDiscardedValueExpr());
 }
 
 /// ParseSEHTryBlockCommon
@@ -958,6 +959,16 @@
   return true;
 }
 
+bool Parser::warnOnDiscardedValueExpr() {
+  if (Actions.getCurCompoundScope().IsStmtExpr) {
+    // 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 +1073,7 @@
         // 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, warnOnDiscardedValueExpr());
       }
     }
 
Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp
+++ lib/Sema/SemaExprCXX.cpp
@@ -7772,7 +7772,7 @@
 }
 
 ExprResult Sema::ActOnFinishFullExpr(Expr *FE, SourceLocation CC,
-                                     bool DiscardedValue,
+                                     bool WarnOnDiscardedValue,
                                      bool IsConstexpr) {
   ExprResult FullExpr = FE;
 
@@ -7782,7 +7782,7 @@
   if (DiagnoseUnexpandedParameterPack(FullExpr.get()))
     return ExprError();
 
-  if (DiscardedValue) {
+  if (WarnOnDiscardedValue) {
     // Top-level expressions default to 'id' when we're in a debugger.
     if (getLangOpts().DebuggerCastResultToId &&
         FullExpr.get()->getType() == Context.UnknownAnyTy) {
@@ -7798,6 +7798,8 @@
     FullExpr = IgnoredValueConversions(FullExpr.get());
     if (FullExpr.isInvalid())
       return ExprError();
+
+    DiagnoseUnusedExprResult(FullExpr.get());
   }
 
   FullExpr = CorrectDelayedTyposInExpr(FullExpr.get());
Index: lib/Sema/SemaOpenMP.cpp
===================================================================
--- lib/Sema/SemaOpenMP.cpp
+++ lib/Sema/SemaOpenMP.cpp
@@ -10243,8 +10243,7 @@
                                          PseudoDstExpr, PseudoSrcExpr);
     if (AssignmentOp.isInvalid())
       continue;
-    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc,
-                                       /*DiscardedValue=*/true);
+    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc);
     if (AssignmentOp.isInvalid())
       continue;
 
@@ -11675,8 +11674,7 @@
                              InitExpr, IV, Step, /* Subtract */ false);
     else
       Update = *CurPrivate;
-    Update = SemaRef.ActOnFinishFullExpr(Update.get(), DE->getBeginLoc(),
-                                         /*DiscardedValue=*/true);
+    Update = SemaRef.ActOnFinishFullExpr(Update.get(), DE->getBeginLoc());
 
     // Build final: Var = InitExpr + NumIterations * Step
     ExprResult Final;
@@ -11686,8 +11684,7 @@
                              InitExpr, NumIterations, Step, /*Subtract=*/false);
     else
       Final = *CurPrivate;
-    Final = SemaRef.ActOnFinishFullExpr(Final.get(), DE->getBeginLoc(),
-                                        /*DiscardedValue=*/true);
+    Final = SemaRef.ActOnFinishFullExpr(Final.get(), DE->getBeginLoc());
 
     if (!Update.isUsable() || !Final.isUsable()) {
       Updates.push_back(nullptr);
@@ -11854,8 +11851,7 @@
                    PseudoSrcExpr);
     if (AssignmentOp.isInvalid())
       continue;
-    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), DE->getExprLoc(),
-                                       /*DiscardedValue=*/true);
+    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), DE->getExprLoc());
     if (AssignmentOp.isInvalid())
       continue;
 
@@ -11963,8 +11959,7 @@
         DSAStack->getCurScope(), ELoc, BO_Assign, PseudoDstExpr, PseudoSrcExpr);
     if (AssignmentOp.isInvalid())
       continue;
-    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc,
-                                       /*DiscardedValue=*/true);
+    AssignmentOp = ActOnFinishFullExpr(AssignmentOp.get(), ELoc);
     if (AssignmentOp.isInvalid())
       continue;
 
Index: lib/Sema/SemaStmt.cpp
===================================================================
--- lib/Sema/SemaStmt.cpp
+++ lib/Sema/SemaStmt.cpp
@@ -42,12 +42,13 @@
 using namespace clang;
 using namespace sema;
 
-StmtResult Sema::ActOnExprStmt(ExprResult FE) {
+StmtResult Sema::ActOnExprStmt(ExprResult FE,
+                               bool WarnOnDiscardedValue) {
   if (FE.isInvalid())
     return StmtError();
 
   FE = ActOnFinishFullExpr(FE.get(), FE.get()->getExprLoc(),
-                           /*DiscardedValue*/ true);
+                           WarnOnDiscardedValue);
   if (FE.isInvalid())
     return StmtError();
 
@@ -371,14 +372,6 @@
       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
@@ -470,15 +463,12 @@
 
 /// 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;
@@ -572,9 +562,6 @@
   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);
 }
@@ -1302,8 +1289,6 @@
       !Diags.isIgnored(diag::warn_comma_operator, CondVal.second->getExprLoc()))
     CommaVisitor(*this).Visit(CondVal.second);
 
-  DiagnoseUnusedExprResult(Body);
-
   if (isa<NullStmt>(Body))
     getCurCompoundScope().setHasEmptyLoopBodies();
 
@@ -1333,8 +1318,6 @@
       !Diags.isIgnored(diag::warn_comma_operator, Cond->getExprLoc()))
     CommaVisitor(*this).Visit(Cond);
 
-  DiagnoseUnusedExprResult(Body);
-
   return new (Context) DoStmt(Body, Cond, DoLoc, WhileLoc, CondRParen);
 }
 
@@ -1779,11 +1762,6 @@
     CommaVisitor(*this).Visit(Second.get().second);
 
   Expr *Third  = third.release().getAs<Expr>();
-
-  DiagnoseUnusedExprResult(First);
-  DiagnoseUnusedExprResult(Third);
-  DiagnoseUnusedExprResult(Body);
-
   if (isa<NullStmt>(Body))
     getCurCompoundScope().setHasEmptyLoopBodies();
 
Index: lib/Sema/TreeTransform.h
===================================================================
--- lib/Sema/TreeTransform.h
+++ lib/Sema/TreeTransform.h
@@ -111,6 +111,8 @@
     }
   };
 
+  unsigned SuppressWarnOnUnusedExpr = 0;
+
 protected:
   Sema &SemaRef;
 
@@ -3292,7 +3294,7 @@
       if (E.isInvalid())
         return StmtError();
 
-      return getSema().ActOnExprStmt(E);
+      return getSema().ActOnExprStmt(E, SuppressWarnOnUnusedExpr == 0);
     }
   }
 
@@ -6518,7 +6520,13 @@
   bool SubStmtChanged = false;
   SmallVector<Stmt*, 8> Statements;
   for (auto *B : S->body()) {
+    bool TransformingLastStmt = B == S->body_back();
+    if (IsStmtExpr && TransformingLastStmt)
+      ++SuppressWarnOnUnusedExpr;
     StmtResult Result = getDerived().TransformStmt(B);
+    if (IsStmtExpr && TransformingLastStmt)
+      --SuppressWarnOnUnusedExpr;
+    
     if (Result.isInvalid()) {
       // Immediately fail if this was a DeclStmt, since it's very
       // likely that this will cause problems for future statements.
@@ -7354,12 +7362,16 @@
 TreeTransform<Derived>::TransformObjCForCollectionStmt(
                                                   ObjCForCollectionStmt *S) {
   // Transform the element statement.
+  ++SuppressWarnOnUnusedExpr;
   StmtResult Element = getDerived().TransformStmt(S->getElement());
+  --SuppressWarnOnUnusedExpr;
   if (Element.isInvalid())
     return StmtError();
 
   // Transform the collection expression.
+  ++SuppressWarnOnUnusedExpr;
   ExprResult Collection = getDerived().TransformExpr(S->getCollection());
+  --SuppressWarnOnUnusedExpr;
   if (Collection.isInvalid())
     return StmtError();
 
Index: test/CXX/stmt.stmt/stmt.select/p3.cpp
===================================================================
--- test/CXX/stmt.stmt/stmt.select/p3.cpp
+++ 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 @@
 // 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}}
 }
Index: test/CodeCompletion/pragma-macro-token-caching.c
===================================================================
--- test/CodeCompletion/pragma-macro-token-caching.c
+++ test/CodeCompletion/pragma-macro-token-caching.c
@@ -12,7 +12,7 @@
 
 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
Index: test/Parser/cxx1z-init-statement.cpp
===================================================================
--- test/Parser/cxx1z-init-statement.cpp
+++ test/Parser/cxx1z-init-statement.cpp
@@ -13,9 +13,9 @@
   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 @@
 
   // 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}}
Index: test/Parser/switch-recovery.cpp
===================================================================
--- test/Parser/switch-recovery.cpp
+++ test/Parser/switch-recovery.cpp
@@ -105,7 +105,7 @@
               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}}
Index: test/SemaCXX/cxx1z-init-statement.cpp
===================================================================
--- test/SemaCXX/cxx1z-init-statement.cpp
+++ 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 @@
     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 @@
       ++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:
Index: test/SemaCXX/for-range-examples.cpp
===================================================================
--- test/SemaCXX/for-range-examples.cpp
+++ test/SemaCXX/for-range-examples.cpp
@@ -176,9 +176,9 @@
 
     // Make sure these don't crash. Better diagnostics would be nice.
     for (: {1, 2, 3}) {} // expected-error {{expected expression}} expected-error {{expected ';'}}
-    for (1 : {1, 2, 3}) {} // expected-error {{must declare a variable}}
+    for (1 : {1, 2, 3}) {} // expected-error {{must declare a variable}} expected-warning {{expression result unused}}
     for (+x : {1, 2, 3}) {} // expected-error {{undeclared identifier}} expected-error {{expected ';'}}
-    for (+y : {1, 2, 3}) {} // expected-error {{must declare a variable}}
+    for (+y : {1, 2, 3}) {} // expected-error {{must declare a variable}} expected-warning {{expression result unused}}
   }
 }
 
@@ -244,7 +244,7 @@
 { 
   int b = 1, a[b];
   a[0] = 0;
-  [&] { for (int c : a) 0; } ();
+  [&] { for (int c : a) 0; } (); // expected-warning {{expression result unused}}
 }
 
 
Index: test/SemaCXX/warn-unused-result.cpp
===================================================================
--- test/SemaCXX/warn-unused-result.cpp
+++ test/SemaCXX/warn-unused-result.cpp
@@ -33,6 +33,36 @@
   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)++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
Index: test/SemaObjCXX/foreach.mm
===================================================================
--- test/SemaObjCXX/foreach.mm
+++ test/SemaObjCXX/foreach.mm
@@ -6,8 +6,8 @@
 void f(NSArray *a) {
     id keys;
     for (int i : a); // expected-error{{selector element type 'int' is not a valid object}} 
-    for ((id)2 : a);  // expected-error {{for range declaration must declare a variable}}
-    for (2 : a); // expected-error {{for range declaration must declare a variable}}
+    for ((id)2 : a);  // expected-error {{for range declaration must declare a variable}} expected-warning {{expression result unused}}
+    for (2 : a); // expected-error {{for range declaration must declare a variable}} expected-warning {{expression result unused}}
   
   for (id thisKey : keys);
 
@@ -71,7 +71,7 @@
 @end
 void test2(NSObject<NSFastEnumeration> *collection) {
   Test2 *obj;
-  for (obj.prop : collection) { // expected-error {{for range declaration must declare a variable}}
+  for (obj.prop : collection) { // expected-error {{for range declaration must declare a variable}} expected-warning {{property access result unused}}
   }
 }