trait TFn1B {
    type In
    type Out
    type Apply[T <: In] <: Out
  }

  trait TFn1[I, O] extends TFn1B {
    type In = I
    type Out = O
  }

  trait >>[F1 <: TFn1[_, _], F2 <: TFn1[_, _]] extends TFn1[F1#In, F2#Out] {
    type Apply[T] = F2#Apply[F1#Apply[T]]
  }

<console>:9: error: illegal inheritance;
 self-type >>[F1,F2] does not conform to TFn1[_$1,_$4]'s selftype TFn1[_$1,_$4]
         trait >>[F1 <: TFn1[_, _], F2 <: TFn1[_, _]] extends TFn1[F1#In, F2#Out] {
                                                              ^
>>[F1,F2] <: TFn1[_$1,_$4]?
  TFn1[_$1,_$4] <: TFn1[_$1,_$4]?
    _$1 <: _$1?
      _$1 <: Nothing?
        <notype> <: Nothing?
        false
        Any <: Nothing?
          <notype> <: Nothing?
          false
        false
      false
      Any <: _$1?
        Any <: Nothing?
          <notype> <: Nothing?
          false
        false
      false
    false
  false
false