Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -554,6 +554,26 @@
       // 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 (MD && MD->getParent()->isRecord()) {
+            MemberSpecializationInfo *NewSI =
+                New->getMemberSpecializationInfo();
+            MemberSpecializationInfo *OldSI =
+                Old->getMemberSpecializationInfo();
+            if (NewSI &&
+                NewSI->getInstantiatedFrom()->getKind() ==
+                    Decl::Kind::CXXConstructor &&
+                NewSI->isExplicitSpecialization() &&
+                OldSI->getTemplateSpecializationKind() ==
+                    TSK_ImplicitInstantiation) {
+              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);