$ cat uncurry-bug.scala 
trait Ctx {
  trait Tree
}

trait Lst[+A] {
  def zip[A1 >: A, B](that: Lst[B]): Nothing
}

object Test {

  /*
   * Question:
   *
   * Is it by design that there is a difference
   * between the behavior of scalac on the following
   * two methods, which only differ in the type param:
   */
  def foo1(c: Ctx)(l: Lst[c.Tree]) = l zip l
  def foo2[T](c: Ctx)(l: Lst[c.Tree]) = l zip l
}

sun@sun-cpu:~/workspace/dev-2.11/scala/sandbox(2.11.x)$ scalac -version
Scala compiler version 2.11.6-20150224-172222-092690e7bf -- Copyright 2002-2013, LAMP/EPFL
sun@sun-cpu:~/workspace/dev-2.11/scala/sandbox(2.11.x)$ scalac uncurry-bug.scala -Xprint:uncurry
[[syntax trees at end of                   uncurry]] // uncurry-bug.scala
package <empty> {
  abstract trait Ctx extends Object {
    def /*Ctx*/$init$(): Unit = {
      ()
    };
    abstract trait Tree extends Object
  };
  abstract trait Lst[+A] extends Object {
    def zip[A1 >: A, B](that: Lst[B]): Nothing
  };
  object Test extends Object {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    def foo1(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)
    };
    def foo2[T](c: Ctx, l: Lst[c.Tree]): Nothing = l.zip[c.Tree, c.Tree](l)
  }
}

def foo1(c: Ctx, l: Lst[Ctx#Tree]): Nothing
def foo2[T](c: Ctx, l: Lst[c.Tree]): Nothing