Add test case

Author: odersky

tests/neg/i2928.scala

@@ -0,0 +1,15 @@
+class Box[T22](var v: T22)
+
+object Test {
+ def main(args: Array[String]): Unit = {
+   val s = new Box[String]("")
+   val i = new Box[Int](3)
+
+   var box: Box[_] = s
+   val sv = box.v
+   box = i
+   box.v = sv // error
+
+   val c: Int = i.v
+ }
+}