Index: clang/lib/Sema/SemaOverload.cpp
===================================================================
--- clang/lib/Sema/SemaOverload.cpp
+++ clang/lib/Sema/SemaOverload.cpp
@@ -13662,11 +13662,32 @@
     CandidateSet.exclude(DefaultedFn);
   LookupOverloadedBinOp(CandidateSet, Op, Fns, Args, PerformADL);
 
-  bool HadMultipleCandidates = (CandidateSet.size() > 1);
-
   // Perform overload resolution.
   OverloadCandidateSet::iterator Best;
-  switch (CandidateSet.BestViableFunction(*this, OpLoc, Best)) {
+  OverloadingResult BestViableFunctionResult =
+      CandidateSet.BestViableFunction(*this, OpLoc, Best);
+
+  // Fallback for C11 atomics in gnu++ mode.
+  if (LangStandard::getLangStandardForKind(getLangOpts().LangStd).isGNUMode()) {
+    // Ignore the atomic keyword on LHS and retry the lookup.
+    if (BestViableFunctionResult == OR_No_Viable_Function) {
+      if (const AtomicType *AtomicTy =
+              dyn_cast<AtomicType>(Args[0]->getType())) {
+        Expr *DeatomizedArg0 =
+            AssertSuccess(ImpCastExprToType(Args[0], AtomicTy->getValueType(),
+                                            CK_AtomicToNonAtomic, VK_LValue));
+        Expr *DeatomizedArgs[2] = {DeatomizedArg0, Args[1]};
+        LookupOverloadedBinOp(CandidateSet, Op, Fns, DeatomizedArgs,
+                              PerformADL);
+        BestViableFunctionResult =
+            CandidateSet.BestViableFunction(*this, OpLoc, Best);
+      }
+    }
+  }
+
+  bool HadMultipleCandidates = (CandidateSet.size() > 1);
+
+  switch (BestViableFunctionResult) {
     case OR_Success: {
       // We found a built-in operator or an overloaded operator.
       FunctionDecl *FnDecl = Best->Function;
Index: clang/test/CodeGenCXX/atomic-builtin-compound-assignment-overload.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/atomic-builtin-compound-assignment-overload.cpp
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -std=gnu++11 -emit-llvm -triple=x86_64-linux-gnu -o - %s | FileCheck %s
+
+_Atomic unsigned an_atomic_uint;
+
+enum { an_enum_value = 1 };
+
+// CHECK-LABEL: define {{.*}}void @_Z5enum1v()
+void enum1() {
+  an_atomic_uint += an_enum_value;
+  // CHECK: atomicrmw add ptr
+}
+
+// CHECK-LABEL: define {{.*}}void @_Z5enum2v()
+void enum2() {
+  an_atomic_uint |= an_enum_value;
+  // CHECK: atomicrmw or ptr
+}
+
+// CHECK-LABEL: define {{.*}}void @_Z5enum3RU7_Atomicj({{.*}})
+void enum3(_Atomic unsigned &an_atomic_uint_param) {
+  an_atomic_uint_param += an_enum_value;
+  // CHECK: atomicrmw add ptr
+}
+
+// CHECK-LABEL: define {{.*}}void @_Z5enum4RU7_Atomicj({{.*}})
+void enum4(_Atomic unsigned &an_atomic_uint_param) {
+  an_atomic_uint_param |= an_enum_value;
+  // CHECK: atomicrmw or ptr
+}
Index: clang/test/SemaCXX/atomic-builtin-compound-assignment-overload.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/atomic-builtin-compound-assignment-overload.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -std=gnu++98 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+_Atomic unsigned an_atomic_uint;
+
+enum { an_enum_value = 1 };
+
+void enum1() { an_atomic_uint += an_enum_value; }
+
+void enum2() { an_atomic_uint |= an_enum_value; }