Fix #9416: Add regression test

Author: OlivierBlanvillain

tests/run/9416.scala

@@ -0,0 +1,20 @@
+trait Base[D <: Domain] {
+  def f(pp: Extract[D]): Unit
+}
+
+class Ext extends Base[ExtDomain] {
+  def f(pp: String) = println(pp.length)
+}
+
+type Domain
+type DomainImpl[T] <: Domain
+type ExtDomain = DomainImpl[String]
+
+type Extract[X] = X match { case DomainImpl[t] => t }
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    def f[M <: Domain](rc: Base[M], v: Extract[M]): Unit =  rc.f(v)
+    f(new Ext, "foo")
+  }
+}