Fix assertion failure in TransformOpaqueValueExpr

[rcraik]

Summary

OpaqueValueExprs may not have a source expression (as in the case when they are created due to a default argument error). This can cause an assertion failure in TransformOpaqueValueExpr during template instantiation.

This patch fixes the assertion failure.

[rcraik]

ping!

[rcraik]

ping 2!

[rcraik]

Ping 3!

[rsmith]

Sorry for the delay, LGTM.

lib/Sema/TreeTransform.h

TreeTransform<Derived>::TransformOffsetOfExpr(OffsetOfExpr *E) {
   return getDerived().RebuildOffsetOfExpr(E->getOperatorLoc(), Type,
                                           Components.data(), Components.size(),
                                           E->getRParenLoc());
 }
 
 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;
 }
 
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformTypoExpr(TypoExpr *E) {
   return E;

test/SemaTemplate/default-arguments.cpp

namespace DR1635 {
   template <class T> struct X {
     template <class U = typename T::type> static void f(int) {} // expected-error {{type 'int' cannot be used prior to '::' because it has no members}} \
                                                                 // expected-warning {{C++11}}
     static void f(...) {}
   };
 
   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;