Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -554,6 +554,20 @@
       // declaration will be instantiated.
       if (New->getFriendObjectKind() == Decl::FOK_None ||
           !New->getLexicalDeclContext()->isDependentContext()) {
+        if (getLangOpts().MicrosoftExt) {
+          // If there is an MS extension make sure that the default argument
+          // that has been instantiated has the right setting before merging.
+          CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(New);
+           if (const auto *spec =
+             dyn_cast<ClassTemplateSpecializationDecl>(MD->getParent())) {
+             if (spec->getTemplateSpecializationKind() ==
+                     TSK_ImplicitInstantiation &&
+                 OldParam->hasUninstantiatedDefaultArg()) {
+               Expr *e = OldParam->getInit();
+               OldParam->setDefaultArg(e);
+             }
+           }
+        }
         // It's important to use getInit() here;  getDefaultArg()
         // strips off any top-level ExprWithCleanups.
         NewParam->setHasInheritedDefaultArg();
Index: clang/test/SemaCXX/dllexport.cpp
===================================================================
--- clang/test/SemaCXX/dllexport.cpp
+++ clang/test/SemaCXX/dllexport.cpp
@@ -1168,3 +1168,13 @@
 // expected-error@+2{{lambda cannot be declared 'dllexport'}}
 #endif
 auto Lambda = []() __declspec(dllexport) -> bool { return true; };
+
+//===----------------------------------------------------------------------===//
+// Constructor Closure with default argument
+//===----------------------------------------------------------------------===//
+template <typename>
+class __declspec(dllexport) foo {
+  foo(int x = 0);
+};
+template <>
+foo<int>::foo(int);