$ cat test.scala 
trait Ctx {
  trait Tree
}
trait Lst[+A] {
  def zip[A1 >: A, B](that: Lst[B]): Nothing
}
object Test {
  def foo(c: Ctx)(l: Lst[c.Tree]) = l zip l
}

$ scalac -version
Scala compiler version 2.11.8-20150821-135328-f8a6d21a49 -- Copyright 2002-2015, LAMP/EPFL

$ scalac test.scala -Xprint:uncurry
[[syntax trees at end of                   uncurry]] // test.scala
package <empty> {
  ...
  object Test extends Object {
    ...
    def foo(c: Ctx, l: Lst[Ctx#Tree]): Nothing = {
      <synthetic> val l$1: Lst[Ctx#Tree] = l.asInstanceOf[Lst[Ctx#Tree]]();
      l$1.zip[c.Tree, c.Tree](l$1)
    }
  }
}

<synthetic> val l$1: Lst[c.Tree] = l.asInstanceOf[Lst[c.Tree]]();

<synthetic> val l$1: Lst[Ctx#Tree] = l.asInstanceOf[Lst[Ctx#Tree]]();

$ scalac test.scala -Ycheck:uncurry
[Now checking: uncurry]

$ cat test-spec.scala 
trait Ctx {
  trait Tree
}
trait Lst[+A] {
  def zip[A1 >: A, B](that: Lst[B]): Nothing
}
class C[@specialized(Int) T] {
  def moo(t: T) = {
    def foo(c: Ctx)(l: Lst[c.Tree]) = l zip l
    ???
  }
}

$ scalac test-spec.scala
test-spec.scala:9: error: type arguments [c.Tree,c.Tree] do not conform to method zip's type parameter bounds [A1 >: Ctx#Tree,B]
    def foo(c: Ctx)(l: Lst[c.Tree]) = l zip l
                                        ^
one error found