add test for #1960

Author: liufengyun

tests/run/i1960.scala

@@ -0,0 +1,8 @@
+case class CC2[A, B](_1: A, _2: B)
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val CC2(_, CC2(a, _)) = CC2(0, CC2(1, 2))
+    assert(a == 1)
+  }
+}