Add xfailed test for #3979

Author: catamorphism

src/test/run-pass/issue-3979.rs

@@ -0,0 +1,12 @@
+// xfail-test
+trait Positioned {
+  fn SetX(int);
+}
+
+trait Movable: Positioned {
+  fn translate(dx: int) {
+    self.SetX(self.X() + dx);
+  }
+}
+
+fn main() {}