Add test that demonstrate that #878 is fixed

It was fixed by 2460f96

Author: smarter

tests/pos/i878.scala

@@ -0,0 +1,17 @@
+class X
+
+class Message[-A]
+class Seg[+A]
+class IChan[A] {
+  def add[B >: A](x: Seg[B])(implicit ev: Message[B]): IChan[B] = ???
+}
+
+class Test {
+  def test: Unit = {
+    implicit val mx: Message[X] = ???
+    val fx: IChan[X] = ???
+    val sx: Seg[X] = ???
+    // the implicit `mx` should be used even though the type parameter of Message is contravariant
+    fx.add(sx)
+  }
+}