Add test: class fields cannot be used as parent prefix

Author: liufengyun

tests/neg/i5083c.scala

@@ -0,0 +1,25 @@
+class A(a: Int) {
+  trait X {
+    def f: Int
+    val x = a + f      // NPE: the outer for `Y` is not yet set
+  }
+
+  trait Y {
+    val y = a
+    def f: Int = A.this.a      // NPE: the outer for `Y` is not yet set
+  }
+
+  class Z(o: A) extends m.Y {  // error
+    val m = o
+  }
+
+  class B extends Z(new A(4))
+}
+
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val a = new A(3)
+    new a.B
+  }
+}