Fix scala#4042: add test

Author: liufengyun

tests/init/neg/i4042.scala

@@ -0,0 +1,18 @@
+object App {
+  def coerce[U, V](u: U): V = {
+    trait X { type R >: U }
+    trait Y { type R = V }
+
+    class T[A <: X](val a: A)(val value: a.R)
+
+    object O { val x : Y & X = x }  // error
+
+    val a = new T[Y & X](O.x)(u)
+    a.value
+  }
+
+  def main(args: Array[String]): Unit = {
+    val x: Int = coerce[String, Int]("a")
+    println(x + 1)
+  }
+}