t5093.scala:2: error: illegal cyclic reference involving type C
  def f[C[X] <: C[X]](l: C[_]) = l.x
                                 ^
t5093.scala:2: error: cyclic aliasing or subtyping involving type C
  def f[C[X] <: C[X]](l: C[_]) = l.x
        ^
t5093.scala:2: error: C does not take type parameters
  def f[C[X] <: C[X]](l: C[_]) = l.x
                         ^
three errors found