diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8562,6 +8562,9 @@
   "'__builtin_choose_expr' requires a constant expression">;
 def warn_unused_expr : Warning<"expression result unused">,
   InGroup<UnusedValue>;
+def warn_unused_comma_left_operand : Warning<
+  "left operand of comma operator has no effect">,
+  InGroup<UnusedValue>;
 def warn_unused_voidptr : Warning<
   "expression result unused; should this cast be to 'void'?">,
   InGroup<UnusedValue>;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -4896,7 +4896,7 @@
 
   /// DiagnoseUnusedExprResult - If the statement passed in is an expression
   /// whose result is unused, warn.
-  void DiagnoseUnusedExprResult(const Stmt *S);
+  void DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID);
   void DiagnoseUnusedNestedTypedefs(const RecordDecl *D);
   void DiagnoseUnusedDecl(const NamedDecl *ND);
 
@@ -5102,6 +5102,16 @@
   /// conversion.
   ExprResult tryConvertExprToType(Expr *E, QualType Ty);
 
+  /// Conditionally issue a diagnostic based on the statement's reachability
+  /// analysis evaluation context.
+  ///
+  /// \param Statement If Statement is non-null, delay reporting the
+  /// diagnostic until the function body is parsed, and then do a basic
+  /// reachability analysis to determine if the statement is reachable.
+  /// If it is unreachable, the diagnostic will not be emitted.
+  bool DiagIfReachable(SourceLocation Loc, ArrayRef<const Stmt *> Stmts,
+                       const PartialDiagnostic &PD);
+
   /// Conditionally issue a diagnostic based on the current
   /// evaluation context.
   ///
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -28,6 +28,7 @@
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/Builtins.h"
+#include "clang/Basic/DiagnosticSema.h"
 #include "clang/Basic/PartialDiagnostic.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
@@ -13369,7 +13370,7 @@
   if (LHS.isInvalid())
     return QualType();
 
-  S.DiagnoseUnusedExprResult(LHS.get());
+  S.DiagnoseUnusedExprResult(LHS.get(), diag::warn_unused_comma_left_operand);
 
   if (!S.getLangOpts().CPlusPlus) {
     RHS = S.DefaultFunctionArrayLvalueConversion(RHS.get());
@@ -18896,6 +18897,37 @@
   EvaluatedExprMarker(*this, SkipLocalVariables).Visit(E);
 }
 
+/// Emit a diagnostic when statements are reachable.
+/// FIXME: check for reachability even in expressions for which we don't build a
+///        CFG (eg, in the initializer of a global or in a constant expression).
+///        For example,
+///        namespace { auto *p = new double[3][false ? (1, 2) : 3]; }
+bool Sema::DiagIfReachable(SourceLocation Loc, ArrayRef<const Stmt *> Stmts,
+                           const PartialDiagnostic &PD) {
+  if (!Stmts.empty() && !FunctionScopes.empty()) {
+    FunctionScopes.back()->PossiblyUnreachableDiags.push_back(
+        sema::PossiblyUnreachableDiag(PD, Loc, Stmts));
+    return true;
+  }
+
+  // The initializer of a constexpr variable or of the first declaration of a
+  // static data member is not syntactically a constant evaluated constant,
+  // but nonetheless is always required to be a constant expression, so we
+  // can skip diagnosing.
+  // FIXME: Using the mangling context here is a hack.
+  if (auto *VD = dyn_cast_or_null<VarDecl>(
+          ExprEvalContexts.back().ManglingContextDecl)) {
+    if (VD->isConstexpr() ||
+        (VD->isStaticDataMember() && VD->isFirstDecl() && !VD->isInline()))
+      return false;
+    // FIXME: For any other kind of variable, we should build a CFG for its
+    // initializer and check whether the context in question is reachable.
+  }
+
+  Diag(Loc, PD);
+  return true;
+}
+
 /// Emit a diagnostic that describes an effect on the run-time behavior
 /// of the program being compiled.
 ///
@@ -18928,28 +18960,7 @@
 
   case ExpressionEvaluationContext::PotentiallyEvaluated:
   case ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed:
-    if (!Stmts.empty() && getCurFunctionOrMethodDecl()) {
-      FunctionScopes.back()->PossiblyUnreachableDiags.
-        push_back(sema::PossiblyUnreachableDiag(PD, Loc, Stmts));
-      return true;
-    }
-
-    // The initializer of a constexpr variable or of the first declaration of a
-    // static data member is not syntactically a constant evaluated constant,
-    // but nonetheless is always required to be a constant expression, so we
-    // can skip diagnosing.
-    // FIXME: Using the mangling context here is a hack.
-    if (auto *VD = dyn_cast_or_null<VarDecl>(
-            ExprEvalContexts.back().ManglingContextDecl)) {
-      if (VD->isConstexpr() ||
-          (VD->isStaticDataMember() && VD->isFirstDecl() && !VD->isInline()))
-        break;
-      // FIXME: For any other kind of variable, we should build a CFG for its
-      // initializer and check whether the context in question is reachable.
-    }
-
-    Diag(Loc, PD);
-    return true;
+    return DiagIfReachable(Loc, Stmts, PD);
   }
 
   return false;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -8524,7 +8524,7 @@
     if (FullExpr.isInvalid())
       return ExprError();
 
-    DiagnoseUnusedExprResult(FullExpr.get());
+    DiagnoseUnusedExprResult(FullExpr.get(), diag::warn_unused_expr);
   }
 
   FullExpr = CorrectDelayedTyposInExpr(FullExpr.get(), /*InitDecl=*/nullptr,
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -216,9 +216,9 @@
   return S.Diag(Loc, diag::warn_unused_result_msg) << A << Msg << R1 << R2;
 }
 
-void Sema::DiagnoseUnusedExprResult(const Stmt *S) {
+void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   if (const LabelStmt *Label = dyn_cast_or_null<LabelStmt>(S))
-    return DiagnoseUnusedExprResult(Label->getSubStmt());
+    return DiagnoseUnusedExprResult(Label->getSubStmt(), DiagID);
 
   const Expr *E = dyn_cast_or_null<Expr>(S);
   if (!E)
@@ -264,7 +264,6 @@
   // Okay, we have an unused result.  Depending on what the base expression is,
   // we might want to make a more specific diagnostic.  Check for one of these
   // cases now.
-  unsigned DiagID = diag::warn_unused_expr;
   if (const FullExpr *Temps = dyn_cast<FullExpr>(E))
     E = Temps->getSubExpr();
   if (const CXXBindTemporaryExpr *TempExpr = dyn_cast<CXXBindTemporaryExpr>(E))
@@ -339,7 +338,7 @@
     if (LangOpts.OpenMP && isa<CallExpr>(Source) &&
         POE->getNumSemanticExprs() == 1 &&
         isa<CallExpr>(POE->getSemanticExpr(0)))
-      return DiagnoseUnusedExprResult(POE->getSemanticExpr(0));
+      return DiagnoseUnusedExprResult(POE->getSemanticExpr(0), DiagID);
     if (isa<ObjCSubscriptRefExpr>(Source))
       DiagID = diag::warn_unused_container_subscript_expr;
     else
@@ -379,7 +378,8 @@
     return;
   }
 
-  DiagRuntimeBehavior(Loc, nullptr, PDiag(DiagID) << R1 << R2);
+  DiagIfReachable(Loc, S ? llvm::makeArrayRef(S) : llvm::None,
+                  PDiag(DiagID) << R1 << R2);
 }
 
 void Sema::ActOnStartOfCompoundStmt(bool IsStmtExpr) {
diff --git a/clang/test/Analysis/dead-stores.c b/clang/test/Analysis/dead-stores.c
--- a/clang/test/Analysis/dead-stores.c
+++ b/clang/test/Analysis/dead-stores.c
@@ -339,12 +339,12 @@
     (void)(0 && x);
     (void)y7;
     (void)(0 || (y8, ({ return; }), 1));
-    // non-nested-warning@-1 {{expression result unused}}
+    // non-nested-warning@-1 {{left operand of comma operator has no effect}}
     (void)x;
     break;
   case 8:
     (void)(1 && (y9, ({ return; }), 1));
-    // non-nested-warning@-1 {{expression result unused}}
+    // non-nested-warning@-1 {{left operand of comma operator has no effect}}
     (void)x;
     break;
   case 9:
diff --git a/clang/test/CXX/basic/basic.link/p8.cpp b/clang/test/CXX/basic/basic.link/p8.cpp
--- a/clang/test/CXX/basic/basic.link/p8.cpp
+++ b/clang/test/CXX/basic/basic.link/p8.cpp
@@ -27,7 +27,7 @@
 void linkage2f(Linkage2);
 
 void use_linkage() {
-  &linkage1v, &linkage1iv, &linkage2v, &linkage2iv, &linkaget1v; // expected-warning 5{{unused}}
+  &linkage1v, &linkage1iv, &linkage2v, &linkage2iv, &linkaget1v; // expected-warning 4{{left operand of comma operator has no effect}} expected-warning {{unused}}
   linkage1f();
   linkage2f({});
 }
diff --git a/clang/test/CXX/drs/dr14xx.cpp b/clang/test/CXX/drs/dr14xx.cpp
--- a/clang/test/CXX/drs/dr14xx.cpp
+++ b/clang/test/CXX/drs/dr14xx.cpp
@@ -18,7 +18,7 @@
       Check<true ? 0 : A::unknown_spec>::type *var1; // expected-error {{undeclared identifier 'var1'}}
       Check<true ? 0 : a>::type *var2; // ok, variable declaration  expected-note 0+{{here}}
       Check<true ? 0 : b>::type *var3; // expected-error {{undeclared identifier 'var3'}}
-      Check<true ? 0 : (c, 0)>::type *var4; // expected-error {{undeclared identifier 'var4'}}
+      Check<true ? 0 : ((void)c, 0)>::type *var4; // expected-error {{undeclared identifier 'var4'}}
       // value-dependent because of the implied type-dependent 'this->', not because of 'd'
       Check<true ? 0 : (d(), 0)>::type *var5; // expected-error {{undeclared identifier 'var5'}}
       // value-dependent because of the value-dependent '&' operator, not because of 'A::d'
diff --git a/clang/test/CXX/drs/dr20xx.cpp b/clang/test/CXX/drs/dr20xx.cpp
--- a/clang/test/CXX/drs/dr20xx.cpp
+++ b/clang/test/CXX/drs/dr20xx.cpp
@@ -221,7 +221,7 @@
         a.*&A::x; // expected-warning {{unused}}
         true ? a.x : a.y; // expected-warning {{unused}}
         (void)a.x;
-        a.x, discarded_lval(); // expected-warning {{unused}}
+        a.x, discarded_lval(); // expected-warning {{left operand of comma operator has no effect}}
 #if 1 // FIXME: These errors are all incorrect; the above code is valid.
       // expected-error@-6 {{enclosing function}}
       // expected-error@-6 {{enclosing function}}
diff --git a/clang/test/CXX/drs/dr7xx.cpp b/clang/test/CXX/drs/dr7xx.cpp
--- a/clang/test/CXX/drs/dr7xx.cpp
+++ b/clang/test/CXX/drs/dr7xx.cpp
@@ -26,12 +26,12 @@
         use(a);
         use((a));
         use(cond ? a : a);
-        use((cond, a)); // expected-warning 2{{unused}} FIXME: should only warn once
+        use((cond, a)); // expected-warning 2{{left operand of comma operator has no effect}} FIXME: should only warn once
 
         (void)a; // FIXME: expected-error {{declared in enclosing}}
         (void)(a); // FIXME: expected-error {{declared in enclosing}}
         (void)(cond ? a : a); // FIXME: expected-error 2{{declared in enclosing}}
-        (void)(cond, a); // FIXME: expected-error {{declared in enclosing}} expected-warning {{unused}}
+        (void)(cond, a); // FIXME: expected-error {{declared in enclosing}} expected-warning {{left operand of comma operator has no effect}}
       }
     };
   }
diff --git a/clang/test/CXX/temp/temp.constr/temp.constr.constr/partial-specializations.cpp b/clang/test/CXX/temp/temp.constr/temp.constr.constr/partial-specializations.cpp
--- a/clang/test/CXX/temp/temp.constr/temp.constr.constr/partial-specializations.cpp
+++ b/clang/test/CXX/temp/temp.constr/temp.constr.constr/partial-specializations.cpp
@@ -26,7 +26,7 @@
   template<typename T> requires (T{}) // expected-error{{atomic constraint must be of type 'bool' (found 'int')}}
   struct B<T**> {};
 
-  static_assert((B<int**>{}, true)); // expected-note{{while checking constraint satisfaction for class template partial specialization 'B<int *>' required here}}
+  static_assert(((void)B<int**>{}, true)); // expected-note{{while checking constraint satisfaction for class template partial specialization 'B<int *>' required here}}
   // expected-note@-1{{while checking constraint satisfaction for class template partial specialization 'B<int>' required here}}
   // expected-note@-2{{during template argument deduction for class template partial specialization 'B<T *>' [with T = int *]}}
   // expected-note@-3{{during template argument deduction for class template partial specialization 'B<T **>' [with T = int]}}
diff --git a/clang/test/CodeCompletion/pragma-macro-token-caching.c b/clang/test/CodeCompletion/pragma-macro-token-caching.c
--- a/clang/test/CodeCompletion/pragma-macro-token-caching.c
+++ b/clang/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; // expected-warning {{expression result unused}}
+    param;
 }
 
 // RUN: %clang_cc1 -fsyntax-only -verify -code-completion-at=%s:16:1 %s | FileCheck %s
diff --git a/clang/test/Frontend/fixed_point_crash.c b/clang/test/Frontend/fixed_point_crash.c
--- a/clang/test/Frontend/fixed_point_crash.c
+++ b/clang/test/Frontend/fixed_point_crash.c
@@ -14,7 +14,7 @@
 int fn2() {
   union a m;
   m.x = 7, 5.6k; // expected-warning {{expression result unused}}
-  return m.x, m.i; // expected-warning {{expression result unused}}
+  return m.x, m.i; // expected-warning {{left operand of comma operator has no effect}}
 }
 
-_Accum acc = (0.5r, 6.9k); // expected-warning {{expression result unused}}
+_Accum acc = (0.5r, 6.9k); // expected-warning {{left operand of comma operator has no effect}}
diff --git a/clang/test/PCH/cxx-explicit-specifier.cpp b/clang/test/PCH/cxx-explicit-specifier.cpp
--- a/clang/test/PCH/cxx-explicit-specifier.cpp
+++ b/clang/test/PCH/cxx-explicit-specifier.cpp
@@ -12,7 +12,7 @@
 
   template<typename X, typename Y> struct T {
     template<typename A>
-    explicit((Y{}, true)) T(A &&a) {}
+    explicit(((void)Y{}, true)) T(A &&a) {}
   };
 
   template<typename X, typename Y> struct U : T<X, Y> {
@@ -28,7 +28,7 @@
 U<S, char> a = foo('0');
 }
 
-//CHECK: explicit((char{} , true))
+//CHECK: explicit(((void)char{} , true))
 
 #endif
 
diff --git a/clang/test/Parser/cxx-ambig-decl-expr.cpp b/clang/test/Parser/cxx-ambig-decl-expr.cpp
--- a/clang/test/Parser/cxx-ambig-decl-expr.cpp
+++ b/clang/test/Parser/cxx-ambig-decl-expr.cpp
@@ -24,7 +24,7 @@
 
   // This is array indexing not an array declarator because a comma expression
   // is not syntactically a constant-expression.
-  int(x[1,1]); // expected-warning 2{{unused}}
+  int(x[1,1]); // expected-warning {{left operand of comma operator has no effect}} expected-warning {{unused}}
 
   // This is array indexing not an array declaration because a braced-init-list
   // is not syntactically a constant-expression.
@@ -36,8 +36,8 @@
   int(a[{0}]); // expected-warning {{unused}}
 
   // These are array declarations.
-  int(x[(1,1)]); // expected-error {{redefinition}}
-  int(x[true ? 1,1 : 1]); // expected-error {{redefinition}}
+  int(x[((void)1,1)]); // expected-error {{redefinition}}
+  int(x[true ? 1 : (1,1)]); // expected-error {{redefinition}} // expected-warning {{left operand of comma operator has no effect}}
 
   int (*_Atomic atomic_ptr_to_int);
   *atomic_ptr_to_int = 42;
diff --git a/clang/test/Parser/cxx0x-ambig.cpp b/clang/test/Parser/cxx0x-ambig.cpp
--- a/clang/test/Parser/cxx0x-ambig.cpp
+++ b/clang/test/Parser/cxx0x-ambig.cpp
@@ -163,7 +163,7 @@
     (void)p1;
 
     UnsignedTmplArgSink<T(CtorSink(t ...)) ...> *t0; // ok
-    UnsignedTmplArgSink<((T *)0, 42u) ...> **t0p = &t0;
+    UnsignedTmplArgSink<((T *)0, 42u) ...> **t0p = &t0; // expected-warning 2{{left operand of comma operator has no effect}}
   }
 
   template void foo(int, int, int); // expected-note {{in instantiation of function template specialization 'ellipsis::foo<int, int>' requested here}}
diff --git a/clang/test/Parser/cxx1z-init-statement.cpp b/clang/test/Parser/cxx1z-init-statement.cpp
--- a/clang/test/Parser/cxx1z-init-statement.cpp
+++ b/clang/test/Parser/cxx1z-init-statement.cpp
@@ -14,8 +14,8 @@
 
   // init-statement expressions
   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}}
+  if (T{f()}, g, h; f()) {} // expected-warning 2{{left operand of comma operator has no effect}} expected-warning {{expression result unused}}
+  if (T(f()), g, h + 1; f()) {} // expected-warning 2{{left operand of comma operator has no effect}} expected-warning {{expression result unused}}
 
   // condition declarations
   if (T(n){g}) {}
@@ -26,8 +26,8 @@
   // condition expressions
   if (T(f())) {}
   if (T{f()}) {}
-  if (T(f()), g, h) {} // expected-warning 2{{unused}}
-  if (T{f()}, g, h) {} // expected-warning 2{{unused}}
+  if (T(f()), g, h) {} // expected-warning 2{{left operand of comma operator has no effect}}
+  if (T{f()}, g, h) {} // expected-warning 2{{left operand of comma operator has no effect}}
 
   // none of the above, disambiguated as expression (can't be a declaration)
   if (T(n)(g)) {} // expected-error {{undeclared identifier 'n'}}
diff --git a/clang/test/Parser/objc-messaging-1.m b/clang/test/Parser/objc-messaging-1.m
--- a/clang/test/Parser/objc-messaging-1.m
+++ b/clang/test/Parser/objc-messaging-1.m
@@ -7,20 +7,20 @@
 	[a ii]; // expected-warning{{not found}}
 	[a if: 1 :2]; // expected-warning{{not found}}
 	[a inout: 1 :2 another:(2,3,4)]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 	[a inout: 1 :2 another:(2,3,4), 6,6,8]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 	[a inout: 1 :2 another:(2,3,4), (6,4,5),6,8]; // expected-warning{{not found}} \
-           // expected-warning 4{{expression result unused}}
+           // expected-warning 4{{left operand of comma operator has no effect}}
 	[a inout: 1 :2 another:(i+10), (i,j-1,5),6,8]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 	[a long: 1 :2 another:(i+10), (i,j-1,5),6,8]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 	[a : "Hello\n" :2 another:(i+10), (i,j-1,5),6,8]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 
 	// Comma expression as receiver (rdar://6222856)
 	[a, b, c foo]; // expected-warning{{not found}} \
-           // expected-warning 2{{expression result unused}}
+           // expected-warning 2{{left operand of comma operator has no effect}}
 
 }
diff --git a/clang/test/Parser/objc-try-catch-1.m b/clang/test/Parser/objc-try-catch-1.m
--- a/clang/test/Parser/objc-try-catch-1.m
+++ b/clang/test/Parser/objc-try-catch-1.m
@@ -28,14 +28,13 @@
     }
     @catch (Frob* ex) {
       @throw 1,2; // expected-error {{@throw requires an Objective-C object type ('int' invalid)}} \
-				  // expected-warning {{expression result unused}}
+				  // expected-warning {{left operand of comma operator has no effect}}
     }
     @catch (float x) {  // expected-error {{@catch parameter is not a pointer to an interface type}}
       
     }
     @catch(...) {
-      @throw (4,3,proc()); // expected-warning {{expression result unused}} \
-						   // expected-warning {{expression result unused}}
+      @throw (4,3,proc()); // expected-warning 2{{left operand of comma operator has no effect}}
     }
   }
 
diff --git a/clang/test/Parser/objcxx11-attributes.mm b/clang/test/Parser/objcxx11-attributes.mm
--- a/clang/test/Parser/objcxx11-attributes.mm
+++ b/clang/test/Parser/objcxx11-attributes.mm
@@ -22,7 +22,7 @@
 
   // A message send which contains a message send is OK.
   [ [ X alloc ] init ];
-  [ [ int(), noreturn getSelf ] getSize ]; // expected-warning {{unused}}
+  [ [ int(), noreturn getSelf ] getSize ]; // expected-warning {{left operand of comma operator has no effect}}
 
   // A message send which contains a lambda is OK.
   [ [noreturn] { return noreturn; } () setSize: 4 ];
diff --git a/clang/test/Sema/const-eval.c b/clang/test/Sema/const-eval.c
--- a/clang/test/Sema/const-eval.c
+++ b/clang/test/Sema/const-eval.c
@@ -74,7 +74,7 @@
 EVAL_EXPR(35, constbool)
 EVAL_EXPR(36, constbool)
 
-EVAL_EXPR(37, (1,2.0) == 2.0 ? 1 : -1)
+EVAL_EXPR(37, ((void)1,2.0) == 2.0 ? 1 : -1)
 EVAL_EXPR(38, __builtin_expect(1,1) == 1 ? 1 : -1)
 
 // PR7884
diff --git a/clang/test/Sema/exprs.c b/clang/test/Sema/exprs.c
--- a/clang/test/Sema/exprs.c
+++ b/clang/test/Sema/exprs.c
@@ -16,7 +16,7 @@
 // This test should be left as is, as it also tests CFG functionality.
 void radar9171946() {
   if (0) {
-    0 / (0 ? 1 : 0); // expected-warning {{expression result unused}}
+    0 / (0 ? 1 : 0); // no-warning
   }
 }
 
diff --git a/clang/test/Sema/i-c-e.c b/clang/test/Sema/i-c-e.c
--- a/clang/test/Sema/i-c-e.c
+++ b/clang/test/Sema/i-c-e.c
@@ -70,10 +70,12 @@
 char z[__builtin_constant_p(4) ? 1 : -1];
 
 // Comma tests
-int comma1[0?1,2:3];
-int comma2[1||(1,2)]; // expected-warning {{use of logical '||' with constant operand}} \
-                      // expected-note {{use '|' for a bitwise operation}}
-int comma3[(1,2)]; // expected-warning {{variable length array folded to constant array as an extension}}
+int comma1[0?1,2:3]; // expected-warning {{left operand of comma operator has no effect}}
+int comma2[1 || (1, 2)]; // expected-warning {{use of logical '||' with constant operand}} \
+                      // expected-note {{use '|' for a bitwise operation}} \
+                      // expected-warning {{left operand of comma operator has no effect}}
+int comma3[(1, 2)];   // expected-warning {{variable length array folded to constant array as an extension}} \
+                      // expected-warning {{left operand of comma operator has no effect}}
 
 // Pointer + __builtin_constant_p
 char pbcp[__builtin_constant_p(4) ? (intptr_t)&expr : 0]; // expected-error {{variable length array declaration not allowed at file scope}}
diff --git a/clang/test/Sema/sizeless-1.c b/clang/test/Sema/sizeless-1.c
--- a/clang/test/Sema/sizeless-1.c
+++ b/clang/test/Sema/sizeless-1.c
@@ -76,9 +76,9 @@
 
   (void)local_int8;
 
-  local_int8, 0; // expected-warning + {{expression result unused}}
+  local_int8, 0; // expected-warning {{left operand of comma operator has no effect}}
 
-  0, local_int8; // expected-warning + {{expression result unused}}
+  0, local_int8; // expected-warning {{left operand of comma operator has no effect}} expected-warning {{expression result unused}}
 
   svint8_t init_int8 = local_int8;
   svint8_t bad_init_int8 = for; // expected-error {{expected expression}}
diff --git a/clang/test/Sema/switch-1.c b/clang/test/Sema/switch-1.c
--- a/clang/test/Sema/switch-1.c
+++ b/clang/test/Sema/switch-1.c
@@ -50,7 +50,7 @@
       return 0;
   }
   return (i, 65537) * 65537; // expected-warning {{overflow in expression; result is 131073 with type 'int'}} \
-			     // expected-warning {{expression result unused}}
+			     // expected-warning {{left operand of comma operator has no effect}}
 }
 
 // rdar://18405357
diff --git a/clang/test/Sema/vla-2.c b/clang/test/Sema/vla-2.c
--- a/clang/test/Sema/vla-2.c
+++ b/clang/test/Sema/vla-2.c
@@ -4,14 +4,14 @@
 // a different codepath when we have already emitted an error.)
 
 int PotentiallyEvaluatedSizeofWarn(int n) {
-  return (int)sizeof *(0 << 32,(int(*)[n])0); // expected-warning {{expression result unused}} expected-warning {{shift count >= width of type}}
+  return (int)sizeof *(0 << 32,(int(*)[n])0); // expected-warning {{left operand of comma operator has no effect}} expected-warning {{shift count >= width of type}}
 }
 
 void PotentiallyEvaluatedTypeofWarn(int n) {
-  __typeof(*(0 << 32,(int(*)[n])0)) x; // expected-warning {{expression result unused}} expected-warning {{shift count >= width of type}}
+  __typeof(*(0 << 32,(int(*)[n])0)) x; // expected-warning {{left operand of comma operator has no effect}} expected-warning {{shift count >= width of type}}
   (void)x;
 }
 
 void PotentiallyEvaluatedArrayBoundWarn(int n) {
-  (void)*(int(*)[(0 << 32,n)])0; // FIXME: We should warn here.
+  (void)*(int(*)[(0 << 32,n)])0; // expected-warning {{left operand of comma operator has no effect}}
 }
diff --git a/clang/test/Sema/warn-type-safety.c b/clang/test/Sema/warn-type-safety.c
--- a/clang/test/Sema/warn-type-safety.c
+++ b/clang/test/Sema/warn-type-safety.c
@@ -145,7 +145,7 @@
 void test_tag_expresssion(int b) {
   fcntl(0, b ? F_DUPFD : F_SETLK, 10); // no-warning
   fcntl(0, b + F_DUPFD, 10); // no-warning
-  fcntl(0, (b, F_DUPFD), 10); // expected-warning {{expression result unused}}
+  fcntl(0, (b, F_DUPFD), 10); // expected-warning {{left operand of comma operator has no effect}}
 }
 
 // Check that using 64-bit magic values as tags works and tag values do not
diff --git a/clang/test/Sema/warn-unused-value.c b/clang/test/Sema/warn-unused-value.c
--- a/clang/test/Sema/warn-unused-value.c
+++ b/clang/test/Sema/warn-unused-value.c
@@ -9,31 +9,31 @@
 
 // PR4806
 void pr4806() {
-  1,foo();          // expected-warning {{expression result unused}}
+  1,foo();          // expected-warning {{left operand of comma operator has no effect}}
 
   // other
   foo();
   i;                // expected-warning {{expression result unused}}
 
-  i,foo();          // expected-warning {{expression result unused}}
+  i,foo();          // expected-warning {{left operand of comma operator has no effect}}
   foo(),i;          // expected-warning {{expression result unused}}
 
-  i,j,foo();        // expected-warning {{expression result unused}} expected-warning {{expression result unused}}
-  i,foo(),j;        // expected-warning {{expression result unused}} expected-warning {{expression result unused}}
-  foo(),i,j;        // expected-warning {{expression result unused}} expected-warning {{expression result unused}}
+  i,j,foo();        // expected-warning 2{{left operand of comma operator has no effect}}
+  i,foo(),j;        // expected-warning {{left operand of comma operator has no effect}} expected-warning {{expression result unused}}
+  foo(),i,j;        // expected-warning {{expression result unused}} expected-warning {{left operand of comma operator has no effect}}
 
   i++;
 
   i++,foo();
   foo(),i++;
 
-  i++,j,foo();      // expected-warning {{expression result unused}}
+  i++,j,foo();      // expected-warning {{left operand of comma operator has no effect}}
   i++,foo(),j;      // expected-warning {{expression result unused}}
   foo(),i++,j;      // expected-warning {{expression result unused}}
 
-  i,j++,foo();      // expected-warning {{expression result unused}}
-  i,foo(),j++;      // expected-warning {{expression result unused}}
-  foo(),i,j++;      // expected-warning {{expression result unused}}
+  i,j++,foo();      // expected-warning {{left operand of comma operator has no effect}}
+  i,foo(),j++;      // expected-warning {{left operand of comma operator has no effect}}
+  foo(),i,j++;      // expected-warning {{left operand of comma operator has no effect}}
 
   i++,j++,foo();
   i++,foo(),j++;
@@ -86,7 +86,7 @@
 void f0(int a);
 void f1(struct s0 *a) {
   // rdar://8139785
-  f0((int)(a->f0 + 1, 10)); // expected-warning {{expression result unused}}
+  f0((int)(a->f0 + 1, 10)); // expected-warning {{left operand of comma operator has no effect}}
 }
 
 void blah(int a);
diff --git a/clang/test/SemaCXX/attr-annotate.cpp b/clang/test/SemaCXX/attr-annotate.cpp
--- a/clang/test/SemaCXX/attr-annotate.cpp
+++ b/clang/test/SemaCXX/attr-annotate.cpp
@@ -42,7 +42,7 @@
 
   template<typename T>
   struct B {
-    [[clang::annotate("test", (T{}, 9))]] void t() {}
+    [[clang::annotate("test", ((void)T{}, 9))]] void t() {}
     // expected-error@-1 {{illegal initializer type 'void'}}
   };
   B<int> b;
@@ -73,7 +73,7 @@
     [[clang::annotate("jui", b, cf)]] void t2() {}
     // expected-error@-1 {{'annotate' attribute requires parameter 1 to be a constant expression}}
     // expected-note@-2 {{is not allowed in a constant expression}}
-    [[clang::annotate("jui", (b, 0), cf)]] [[clang::annotate("jui", &b, cf, &foo::t2, str())]] void t3() {}
+    [[clang::annotate("jui", ((void)b, 0), cf)]] [[clang::annotate("jui", &b, cf, &foo::t2, str())]] void t3() {}
   };
 };
 
diff --git a/clang/test/SemaCXX/builtin-constant-p.cpp b/clang/test/SemaCXX/builtin-constant-p.cpp
--- a/clang/test/SemaCXX/builtin-constant-p.cpp
+++ b/clang/test/SemaCXX/builtin-constant-p.cpp
@@ -157,12 +157,12 @@
     constexpr ~A() { *p = 0; }
   };
   struct Q { int n; constexpr int *get() { return &n; } };
-  static_assert(!__builtin_constant_p((A{}, 123)));
+  static_assert(!__builtin_constant_p(((void)A{}, 123)));
   // FIXME: We should probably accept this. GCC does.
   // However, GCC appears to do so by running the destructors at the end of the
   // enclosing full-expression, which seems broken; running them at the end of
   // the evaluation of the __builtin_constant_p argument would be more
   // defensible.
-  static_assert(!__builtin_constant_p((A{Q().get()}, 123)));
+  static_assert(!__builtin_constant_p(((void)A{Q().get()}, 123)));
 }
 #endif
diff --git a/clang/test/SemaCXX/constant-expression-cxx2a.cpp b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
--- a/clang/test/SemaCXX/constant-expression-cxx2a.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
@@ -745,7 +745,7 @@
   // Ensure that we can handle temporary cleanups for array temporaries.
   struct ArrElem { constexpr ~ArrElem() {} };
   using Arr = ArrElem[3];
-  static_assert((Arr{}, true));
+  static_assert(((void)Arr{}, true));
 }
 
 namespace dynamic_alloc {
diff --git a/clang/test/SemaCXX/constant-expression.cpp b/clang/test/SemaCXX/constant-expression.cpp
--- a/clang/test/SemaCXX/constant-expression.cpp
+++ b/clang/test/SemaCXX/constant-expression.cpp
@@ -88,8 +88,8 @@
 
 void diags(int n) {
   switch (n) {
-    case (1/0, 1): // expected-error {{not an integral constant expression}} expected-note {{division by zero}}
-    case (int)(1/0, 2.0): // expected-error {{not an integral constant expression}} expected-note {{division by zero}}
+    case (1/0, 1): // expected-error {{not an integral constant expression}} expected-note {{division by zero}} expected-warning {{left operand of comma operator has no effect}}
+    case (int)(1/0, 2.0): // expected-error {{not an integral constant expression}} expected-note {{division by zero}} expected-warning {{left operand of comma operator has no effect}}
     case __imag(1/0): // expected-error {{not an integral constant expression}} expected-note {{division by zero}}
     case (int)__imag((double)(1/0)): // expected-error {{not an integral constant expression}} expected-note {{division by zero}}
       ;
diff --git a/clang/test/SemaCXX/expression-traits.cpp b/clang/test/SemaCXX/expression-traits.cpp
--- a/clang/test/SemaCXX/expression-traits.cpp
+++ b/clang/test/SemaCXX/expression-traits.cpp
@@ -583,10 +583,10 @@
 
     // Can't use the ASSERT_XXXX macros without adding parens around
     // the comma expression.
-    static_assert(__is_lvalue_expr(x,x), "expected an lvalue");
-    static_assert(__is_rvalue_expr(x,1), "expected an rvalue");
-    static_assert(__is_lvalue_expr(1,x), "expected an lvalue");
-    static_assert(__is_rvalue_expr(1,1), "expected an rvalue");
+    static_assert(__is_lvalue_expr((void)x,x), "expected an lvalue");
+    static_assert(__is_rvalue_expr((void)x,1), "expected an rvalue");
+    static_assert(__is_lvalue_expr((void)1,x), "expected an lvalue");
+    static_assert(__is_rvalue_expr((void)1,1), "expected an rvalue");
 }
 
 #if 0
diff --git a/clang/test/SemaCXX/matrix-type-operators.cpp b/clang/test/SemaCXX/matrix-type-operators.cpp
--- a/clang/test/SemaCXX/matrix-type-operators.cpp
+++ b/clang/test/SemaCXX/matrix-type-operators.cpp
@@ -179,12 +179,12 @@
 
   a[4, 5] = 5.0;
   // expected-error@-1 {{comma expressions are not allowed as indices in matrix subscript expressions}}
-  // expected-warning@-2 {{expression result unused}}
+  // expected-warning@-2 {{left operand of comma operator has no effect}}
 
   a[4, 5, 4] = 5.0;
   // expected-error@-1 {{comma expressions are not allowed as indices in matrix subscript expressions}}
-  // expected-warning@-2 {{expression result unused}}
-  // expected-warning@-3 {{expression result unused}}
+  // expected-warning@-2 {{left operand of comma operator has no effect}}
+  // expected-warning@-3 {{left operand of comma operator has no effect}}
 }
 
 void extract(sx5x10_t a, float f) {
diff --git a/clang/test/SemaCXX/overloaded-operator.cpp b/clang/test/SemaCXX/overloaded-operator.cpp
--- a/clang/test/SemaCXX/overloaded-operator.cpp
+++ b/clang/test/SemaCXX/overloaded-operator.cpp
@@ -157,7 +157,7 @@
 
 void test_comma(X x, Y y) {
   bool& b1 = (x, y);
-  X& xr = (x, x); // expected-warning {{expression result unused}}
+  X& xr = (x, x); // expected-warning {{left operand of comma operator has no effect}}
 }
 
 struct Callable {
diff --git a/clang/test/SemaCXX/sizeless-1.cpp b/clang/test/SemaCXX/sizeless-1.cpp
--- a/clang/test/SemaCXX/sizeless-1.cpp
+++ b/clang/test/SemaCXX/sizeless-1.cpp
@@ -85,9 +85,9 @@
 
   (void)local_int8;
 
-  local_int8, 0; // expected-warning + {{expression result unused}}
+  local_int8, 0; // expected-warning {{left operand of comma operator has no effect}}
 
-  0, local_int8; // expected-warning + {{expression result unused}}
+  0, local_int8; // expected-warning {{left operand of comma operator has no effect}} expected-warning {{expression result unused}}
 
   svint8_t init_int8 = local_int8;
   svint8_t bad_init_int8 = for; // expected-error {{expected expression}}
diff --git a/clang/test/SemaCXX/vector.cpp b/clang/test/SemaCXX/vector.cpp
--- a/clang/test/SemaCXX/vector.cpp
+++ b/clang/test/SemaCXX/vector.cpp
@@ -381,8 +381,8 @@
 typedef int inte2 __attribute__((__ext_vector_type__(2)));
 
 void test_vector_literal(inte4 res) {
-  inte2 a = (inte2)(1, 2); //expected-warning{{expression result unused}}
-  inte4 b = (inte4)(a, a); //expected-error{{C-style cast from vector 'inte2' (vector of 2 'int' values) to vector 'inte4' (vector of 4 'int' values) of different size}} //expected-warning{{expression result unused}}
+  inte2 a = (inte2)(1, 2); //expected-warning{{left operand of comma operator has no effect}}
+  inte4 b = (inte4)(a, a); //expected-error{{C-style cast from vector 'inte2' (vector of 2 'int' values) to vector 'inte4' (vector of 4 'int' values) of different size}} //expected-warning{{left operand of comma operator has no effect}}
 }
 
 typedef __attribute__((__ext_vector_type__(4))) float vector_float4;
diff --git a/clang/test/SemaCXX/warn-comma-operator.cpp b/clang/test/SemaCXX/warn-comma-operator.cpp
--- a/clang/test/SemaCXX/warn-comma-operator.cpp
+++ b/clang/test/SemaCXX/warn-comma-operator.cpp
@@ -242,8 +242,8 @@
 
 template <typename... xs>
 class Foo {
-  typedef bool_seq<(xs::value, true)...> all_true;
-  typedef bool_seq<(xs::value, false)...> all_false;
+  typedef bool_seq<((void)xs::value, true)...> all_true;
+  typedef bool_seq<((void)xs::value, false)...> all_false;
   typedef bool_seq<xs::value...> seq;
 };
 
diff --git a/clang/test/SemaCXX/warn-unused-value.cpp b/clang/test/SemaCXX/warn-unused-value.cpp
--- a/clang/test/SemaCXX/warn-unused-value.cpp
+++ b/clang/test/SemaCXX/warn-unused-value.cpp
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wunused-value %s
 // RUN: %clang_cc1 -fsyntax-only -verify -Wunused-value -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -Wunused-value -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wunused-value -std=c++17 %s
 
 // PR4806
 namespace test0 {
@@ -138,3 +139,26 @@
   (void)arr3;
   (void)arr4;
 }
+
+#if __cplusplus >= 201103L // C++11 or later
+namespace test5 {
+int v[(5, 6)]; // expected-warning {{left operand of comma operator has no effect}}
+void foo() {
+  new double[false ? (1, 2) : 3]
+            // FIXME: We shouldn't diagnose the unreachable constant expression
+            // here.
+            [false ? (1, 2) : 3]; // expected-warning {{left operand of comma operator has no effect}}
+}
+} // namespace test5
+#endif
+
+#if __cplusplus >= 201703L // C++11 or later
+namespace test6 {
+auto b() {
+  if constexpr (false)
+    return (1,0);
+  else
+    return (1.0,0.0); // expected-warning {{left operand of comma operator has no effect}}
+}
+} // namespace test6
+#endif
diff --git a/clang/test/SemaTemplate/derived.cpp b/clang/test/SemaTemplate/derived.cpp
--- a/clang/test/SemaTemplate/derived.cpp
+++ b/clang/test/SemaTemplate/derived.cpp
@@ -49,6 +49,6 @@
 
   class A {
     TFP m_p;
-    void Enable() { 0, A(); } // expected-warning {{unused}}
+    void Enable() { 0, A(); } // expected-warning {{left operand of comma operator has no effect}}
   };
 }
diff --git a/clang/test/SemaTemplate/lambda-capture-pack.cpp b/clang/test/SemaTemplate/lambda-capture-pack.cpp
--- a/clang/test/SemaTemplate/lambda-capture-pack.cpp
+++ b/clang/test/SemaTemplate/lambda-capture-pack.cpp
@@ -18,7 +18,7 @@
 namespace PR41576 {
   template <class... Xs> constexpr int f(Xs ...xs) {
     return [&](auto ...ys) { // expected-note {{instantiation}}
-      return ((xs + ys), ...); // expected-warning {{unused}}
+      return ((xs + ys), ...); // expected-warning {{left operand of comma operator has no effect}}
     }(1, 2);
   }
   static_assert(f(3, 4) == 6); // expected-note {{instantiation}}