I have the following code:
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]]
}
It compiles on Scala 2.9.1 without a warning or an error.
But on the current 2.10 build, I get the following error message:
Fun.scala:12: error: illegal inheritance;
self-type this.>>[F1,F2] does not conform to this.TFn1[_$1,_$4]'s selftype this.TFn1[_$1,_$4]
trait >>[F1 <: TFn1[_, _], F2 <: TFn1[_, _]] extends TFn1[F1#In, F2#Out] {
^
one error found
Is this a regression or is the code unsound and the compiler just started to catch it lately? If the code is wrong, what would be a work-around to make it work again?
It looks like a bug to me. There's no constraints on the type parameters of the super classes, but, more to the point, the trace for -explaintypes looks suspicious to me:
scala> 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
Specifically, I don't understand how or why it cannot prove that TFn1[_$1,_$4] <: TFn1[_$1,_$4] or even _$1 <: _$1.