Add test case

Author: odersky

tests/pos/i1757.scala

@@ -0,0 +1,6 @@
+case class B[T](b: List[Int]) {
+  var s: B[Int] = ???
+  def cpy[X](b: List[Int] = b): B[X] = new B[X](b)
+  s.cpy()
+  s.copy()
+}