Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -5862,9 +5862,21 @@
         // Instantiate non-default class member functions ...
 
         // .. except for certain kinds of template specializations.
-        if (TSK == TSK_ImplicitInstantiation && !ClassAttr->isInherited())
+        if (TSK == TSK_ImplicitInstantiation && !ClassAttr->isInherited()) {
+          // But before that, make sure to instantiate default args for
+          // defaulted constructor.
+          CXXConstructorDecl *Ctor = dyn_cast<CXXConstructorDecl>(Member);
+          if (Ctor && Ctor->isDefaultConstructor() &&
+              Ctor->hasOneParamOrDefaultArgs()) {
+            for (unsigned I = 0; I != Ctor->getNumParams(); ++I) {
+              S.CheckCXXDefaultArgExpr(
+                  Member->getAttr<DLLExportAttr>()->getLocation(), Ctor,
+                  Ctor->getParamDecl(I));
+              S.DiscardCleanupsInEvaluationContext();
+            }
+          }
           continue;
-
+        }
         S.MarkFunctionReferenced(Class->getLocation(), MD);
 
         // The function will be passed to the consumer when its definition is
Index: clang/test/SemaCXX/default-arg-closures.cpp
===================================================================
--- clang/test/SemaCXX/default-arg-closures.cpp
+++ clang/test/SemaCXX/default-arg-closures.cpp
@@ -42,3 +42,10 @@
 void f(const Default &d) {
   throw d;
 }
+
+template <typename>
+class __declspec(dllexport) foo {
+  foo(int x = 0);
+};
+template <>
+foo<int>::foo(int);