* tree.c (walk_tree): Don't walk into default args.

From-SVN: r36985

gcc/testsuite/g++.old-deja/g++.other/inline15.C

+// Build don't link:
+// Origin: Jakub Jelinek <jakub@redhat.com>
+// Special g++ Options: -O1
+
+class Type;
+template<class E>
+class X
+{
+public:
+  X<E>();
+  inline X<E>(int);
+  inline ~X<E>();
+};
+template<class E> const Type &foo(const X<E> *);
+template<class E> inline X<E>::X(int x)
+{
+  const Type &a = foo(this);
+}
+template<class E> inline X<E>::~X()
+{
+  const Type &a = foo(this);
+}
+class Y
+{
+  X<Type> a;
+public:
+  Y(const X<Type> &x = X<Type>());
+};
+Y::Y(const X<Type> &x) : a(1)
+{
+}