add another test for i1540

Author: liufengyun

tests/pos/i1540b.scala

@@ -0,0 +1,14 @@
+class Casey1[T](val a: T) {
+  def isDefined: Boolean = true
+  def isDefined(x: T): Boolean = ???
+  def get: T = a
+  def get(x: T): String = ???
+}
+object Casey1 { def unapply[T](a: Casey1[T]) = a }
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val c @ Casey1(x) = new Casey1(0)
+    assert(x == c.get)
+  }
+}