Fix #3139: Add regression test

Author: odersky

tests/pos/i3139.scala

@@ -0,0 +1,11 @@
+trait Foo[T] {
+    type Base[A]
+}
+
+object Test {
+  def foo[T](ev: Foo[T]): Foo[T] { type Base[A] = ev.Base[A] } = ev
+}
+
+object Test2 {
+  def foo[T](ev: Foo[T]): Foo[T] { type Base = ev.Base } = ev
+}