Index: lib/Sema/TreeTransform.h
===================================================================
--- lib/Sema/TreeTransform.h
+++ lib/Sema/TreeTransform.h
@@ -7730,7 +7730,7 @@
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformOpaqueValueExpr(OpaqueValueExpr *E) {
-  assert(getDerived().AlreadyTransformed(E->getType()) &&
+  assert((!E->getSourceExpr() || getDerived().AlreadyTransformed(E->getType())) &&
          "opaque value expression requires transformation");
   return E;
 }
Index: test/SemaTemplate/default-arguments.cpp
===================================================================
--- test/SemaTemplate/default-arguments.cpp
+++ test/SemaTemplate/default-arguments.cpp
@@ -159,3 +159,9 @@
 
   int g() { X<int>::f(0); } // expected-note {{in instantiation of template class 'DR1635::X<int>' requested here}}
 }
+
+template <typename T>
+struct C {
+  C(T t = ); // expected-error {{expected expression}}
+};
+C<int> obj;