Index: lib/Sema/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp
+++ lib/Sema/SemaDeclAttr.cpp
@@ -6485,6 +6485,16 @@
       D->setInvalidDecl();
     }
   }
+  //In Microsoft C++ mode a const variables defined in namespace scope has external
+  // linkage by default if the variable is declared with __declspec(dllexport).
+  if (auto *VD = dyn_cast<VarDecl>(D)) {
+    if (getLangOpts().CPlusPlus && getLangOpts().MSVCCompat &&
+        VD->getType().isLocalConstQualified() &&
+        VD->hasAttr<DLLExportAttr>() &&
+        VD->getDefinition()) {
+      VD->setStorageClass(SC_Extern);
+    }
+  }
 }
 
 // Helper for delayed processing TransparentUnion attribute.
Index: test/Sema/dllexport.c
===================================================================
--- test/Sema/dllexport.c
+++ test/Sema/dllexport.c
@@ -66,6 +66,8 @@
   __declspec(dllexport) static int StaticLocalVar; // expected-error{{'StaticLocalVar' must have external linkage when declared 'dllexport'}}
 }
 
+// const variables
+__declspec(dllexport) int const x = 3;
 
 
 //===----------------------------------------------------------------------===//