Merge pull request scala#591 from retronym/ticket/3888

Test case closes SI-3880.

Author: Adriaan Moors

test/files/pos/t3880.scala

@@ -0,0 +1,16 @@
+abstract class Bar[+B] {
+}
+abstract class C1[+B] extends Bar[B] {  
+  private[this] def g(x: C1[B]): Unit = ()
+
+  // this method is fine: notice that it allows the call to g,
+  // which requires C1[B], even though we matched on C1[_].
+  // (That is good news.)
+  private[this] def f1(x: Bar[B]): Unit = x match {
+    case x: C1[_] => g(x)
+  }
+  // this one crashes.
+  private[this] def f2(x: Bar[B]): Unit = x match {
+    case x: C1[_] => f2(x)
+  }
+}