Fix #6126: Add regression test

Author: nicolasstucki

tests/pos/i6126.scala

@@ -0,0 +1,9 @@
+class Test {
+  class Foo[+T](val x: T)
+
+  def unfoo[T](foo: Foo[T]): T = foo.x
+
+  def fooArray: Foo[Array[Object]] = new Foo(Array.empty[Object])
+
+  unfoo(fooArray) ++ unfoo(fooArray)
+}