Mistake in subkinding

[scabug]

trait A[B[D[X, Y <: X]]] {
 def f() : B[P]
}

trait C[E[T, S]]

trait P[U, V <: U]

Now, the kinds are:

D : X@* -> *(Nothing,X) -> *
B : (X@* -> *(Nothing,X) -> *) -> *

E : * -> * -> *
C : (* -> * -> *) -> *

In type application A[C], C is substituted instead of the type parameter B, which leads to the ill-kinded type application C[P] in the return type of f().

So (* -> * -> *) -> * must be a subkind of (X@* -> *(Nothing,X) -> *) -> *.
Contravariantly, X@* -> *(Nothing,X) -> * must be a subkind of * -> * -> *.
Contravariantly, * must be a subkind of *(Nothing,X), but this is not the case.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2067?orig=1
Reporter: Vladimir Reshetnikov (nikov)

[scabug]

Vladimir Reshetnikov (nikov) said:
A similar problem exists for variance annotations:

class P[Y](val y : Y)

trait A[T[_[_]]] {
  def g() : T[P] = error("")
}

trait C[X[+_]] {
  def f(x : X[Int]) : X[Any] = x
}

Type application A[C] should be rejected as ill-kinded, because C[P] is ill-kinded.