Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6723,6 +6723,7 @@
 def err_cuda_vla : Error<
     "cannot use variable-length arrays in "
     "%select{__device__|__global__|__host__|__host__ __device__}0 functions">;
+def err_cuda_extern_shared : Error<"__shared__ variable %0 cannot be 'extern'">;
 
 def warn_non_pod_vararg_with_format_string : Warning<
   "cannot pass %select{non-POD|non-trivial}0 object of type %1 to variadic "
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -3696,6 +3696,19 @@
     D->addAttr(Optnone);
 }
 
+static void handleSharedAttr(Sema &S, Decl *D, const AttributeList &Attr) {
+  if (checkAttrMutualExclusion<CUDASharedAttr>(S, D, Attr.getRange(),
+                                               Attr.getName()))
+    return;
+  auto *VD = cast<VarDecl>(D);
+  if (VD->hasExternalStorage()) {
+    S.Diag(Attr.getLoc(), diag::err_cuda_extern_shared) << VD;
+    return;
+  }
+  D->addAttr(::new (S.Context) CUDASharedAttr(
+      Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex()));
+}
+
 static void handleGlobalAttr(Sema &S, Decl *D, const AttributeList &Attr) {
   if (checkAttrMutualExclusion<CUDADeviceAttr>(S, D, Attr.getRange(),
                                                Attr.getName()) ||
@@ -5639,8 +5652,7 @@
     handleSimpleAttribute<NoThrowAttr>(S, D, Attr);
     break;
   case AttributeList::AT_CUDAShared:
-    handleSimpleAttributeWithExclusions<CUDASharedAttr, CUDAConstantAttr>(S, D,
-                                                                          Attr);
+    handleSharedAttr(S, D, Attr);
     break;
   case AttributeList::AT_VecReturn:
     handleVecReturnAttr(S, D, Attr);
Index: clang/test/SemaCUDA/extern-shared.cu
===================================================================
--- /dev/null
+++ clang/test/SemaCUDA/extern-shared.cu
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -fcuda-is-device -verify %s
+
+#include "Inputs/cuda.h"
+
+__device__ void foo() {
+  extern __shared__ int x; // expected-error {{__shared__ variable 'x' cannot be 'extern'}}
+}
+
+__host__ __device__ void bar() {
+  extern __shared__ int x; // expected-error {{__shared__ variable 'x' cannot be 'extern'}}
+}
+
+extern __shared__ int global; // expected-error {{__shared__ variable 'global' cannot be 'extern'}}