Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -9253,6 +9253,8 @@
                           LateInstantiatedAttrVec *LateAttrs = nullptr,
                           LocalInstantiationScope *OuterMostScope = nullptr);
 
+  void InstantiateDefaultCtorDefaultArgs(CXXConstructorDecl *Ctor);
+
   bool usesPartialOrExplicitSpecialization(
       SourceLocation Loc, ClassTemplateSpecializationDecl *ClassTemplateSpec);
 
Index: clang/lib/CodeGen/MicrosoftCXXABI.cpp
===================================================================
--- clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1244,12 +1244,14 @@
   // the typical calling convention and have a single 'this' pointer for an
   // argument -or- they get a wrapper function which appropriately thunks to the
   // real default constructor.  This thunk is the default constructor closure.
-  if (D->hasAttr<DLLExportAttr>() && D->isDefaultConstructor())
+  if (D->hasAttr<DLLExportAttr>() && D->isDefaultConstructor() &&
+      D->isDefined()) {
     if (!hasDefaultCXXMethodCC(getContext(), D) || D->getNumParams() != 0) {
       llvm::Function *Fn = getAddrOfCXXCtorClosure(D, Ctor_DefaultClosure);
       Fn->setLinkage(llvm::GlobalValue::WeakODRLinkage);
       CGM.setGVProperties(Fn, D);
     }
+  }
 }
 
 void MicrosoftCXXABI::EmitVBPtrStores(CodeGenFunction &CGF,
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13994,6 +13994,16 @@
     FD->setInvalidDecl();
   }
 
+  if (auto *Ctor = dyn_cast<CXXConstructorDecl>(FD)) {
+    if (Ctor->getTemplateSpecializationKind() == TSK_ExplicitSpecialization &&
+        Ctor->isDefaultConstructor() &&
+        Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+      // If this is an MS ABI dllexport default constructor, instantiate any
+      // default arguments.
+      InstantiateDefaultCtorDefaultArgs(Ctor);
+    }
+  }
+
   // See if this is a redefinition. If 'will have body' (or similar) is already
   // set, then these checks were already performed when it was set.
   if (!FD->willHaveBody() && !FD->isLateTemplateParsed() &&
Index: clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -715,6 +715,26 @@
   }
 }
 
+/// In the MS ABI, we need to instantiate default arguments of dllexported
+/// default constructors along with the constructor definition. This allows IR
+/// gen to emit a constructor closure which calls the default constructor with
+/// its default arguments.
+void Sema::InstantiateDefaultCtorDefaultArgs(CXXConstructorDecl *Ctor) {
+  assert(Context.getTargetInfo().getCXXABI().isMicrosoft() &&
+         Ctor->isDefaultConstructor());
+  unsigned NumParams = Ctor->getNumParams();
+  if (NumParams == 0)
+    return;
+  DLLExportAttr *Attr = Ctor->getAttr<DLLExportAttr>();
+  if (!Attr)
+    return;
+  for (unsigned I = 0; I != NumParams; ++I) {
+    (void)CheckCXXDefaultArgExpr(Attr->getLocation(), Ctor,
+                                   Ctor->getParamDecl(I));
+    DiscardCleanupsInEvaluationContext();
+  }
+}
+
 /// Get the previous declaration of a declaration for the purposes of template
 /// instantiation. If this finds a previous declaration, then the previous
 /// declaration of the instantiation of D should be an instantiation of the
@@ -4608,27 +4628,6 @@
   return cast_or_null<FunctionDecl>(SubstDecl(FD, FD->getParent(), MArgs));
 }
 
-/// In the MS ABI, we need to instantiate default arguments of dllexported
-/// default constructors along with the constructor definition. This allows IR
-/// gen to emit a constructor closure which calls the default constructor with
-/// its default arguments.
-static void InstantiateDefaultCtorDefaultArgs(Sema &S,
-                                              CXXConstructorDecl *Ctor) {
-  assert(S.Context.getTargetInfo().getCXXABI().isMicrosoft() &&
-         Ctor->isDefaultConstructor());
-  unsigned NumParams = Ctor->getNumParams();
-  if (NumParams == 0)
-    return;
-  DLLExportAttr *Attr = Ctor->getAttr<DLLExportAttr>();
-  if (!Attr)
-    return;
-  for (unsigned I = 0; I != NumParams; ++I) {
-    (void)S.CheckCXXDefaultArgExpr(Attr->getLocation(), Ctor,
-                                   Ctor->getParamDecl(I));
-    S.DiscardCleanupsInEvaluationContext();
-  }
-}
-
 /// Instantiate the definition of the given function from its
 /// template.
 ///
@@ -4847,7 +4846,7 @@
         // default arguments.
         if (Context.getTargetInfo().getCXXABI().isMicrosoft() &&
             Ctor->isDefaultConstructor()) {
-          InstantiateDefaultCtorDefaultArgs(*this, Ctor);
+          InstantiateDefaultCtorDefaultArgs(Ctor);
         }
       }
 
Index: clang/test/CodeGenCXX/dllexport-ctor-closure.cpp
===================================================================
--- clang/test/CodeGenCXX/dllexport-ctor-closure.cpp
+++ clang/test/CodeGenCXX/dllexport-ctor-closure.cpp
@@ -50,6 +50,14 @@
 // CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_F?$TemplateWithClosure@H@@QAEXXZ"({{.*}}) {{#[0-9]+}} comdat
 // CHECK:   call {{.*}} @"??0?$TemplateWithClosure@H@@QAE@H@Z"({{.*}}, i32 4)
 
+template <typename T> struct __declspec(dllexport) ExportedTemplateWithClosure {
+  ExportedTemplateWithClosure(int x = sizeof(T)) {}
+};
+template <> ExportedTemplateWithClosure<int>::ExportedTemplateWithClosure(int); // Don't try to emit the closure for a declaration.
+template <> ExportedTemplateWithClosure<int>::ExportedTemplateWithClosure(int) {};
+// CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_F?$ExportedTemplateWithClosure@H@@QAEXXZ"({{.*}}) {{#[0-9]+}} comdat
+// CHECK:   call {{.*}} @"??0?$ExportedTemplateWithClosure@H@@QAE@H@Z"({{.*}}, i32 4)
+
 struct __declspec(dllexport) NestedOuter {
   DELETE_IMPLICIT_MEMBERS(NestedOuter);
   NestedOuter(void *p = 0) {}
Index: clang/test/SemaCXX/dllexport.cpp
===================================================================
--- clang/test/SemaCXX/dllexport.cpp
+++ clang/test/SemaCXX/dllexport.cpp
@@ -792,6 +792,11 @@
 };
 template struct HasDefaults2<void>; // expected-note {{in instantiation of member function 'HasDefaults2<void>::HasDefaults2' requested here}}
 
+template <typename T> struct __declspec(dllexport) HasDefaults3 { // expected-note{{in instantiation of default function argument expression for 'HasDefaults3<void>' required here}}
+  HasDefaults3(int x = sizeof(T)) {} // expected-error {{invalid application of 'sizeof'}}
+};
+template <> HasDefaults3<void>::HasDefaults3(int) {};
+
 #endif
 
 //===----------------------------------------------------------------------===//