Given
trait Int // proper type
trait List[A] // 1st-order-kinded type constructor
trait Functor[F[_]] // higher-order-kinded type constructor taking type constructor
trait I[H[F[_]]] // higher-order-kinded type constructor taking higher-order type constructor that takes 1st-order type constructor
we cannot pass type argument of different kind as compared to the kind of declared type parameter
scala> def f[F[_[_[_]]]] = 42
def f[F[_$1]] => Int
scala> f[I]
val res5: Int = 42
scala> f[Functor]
1 |f[Functor]
| ^
| Type argument Functor does not conform to upper bound [_$1[_$2]] =>> Any
however we can declare the type parameter to be polymorphic in its kind via AnyKind
scala> def f[A <: AnyKind] = 42
def f[A <: AnyKind] => Int
scala> f[Int]
val res10: Int = 42
scala> f[List]
val res11: Int = 42
scala> f[Functor]
val res12: Int = 42
scala> f[I]
val res13: Int = 42
What is the use case of AnyKind? What practical problem does it solve?
AnyKindis to index types and type classes that are naturally kind polymorphic, such as Type in the Scala 3 quote/splice based metaprogramming facility. – Seligman