scala3-boundtype.scala

import scala.compiletime.ops.int.*

// Only allowed values are Min <= N <= Max.
type Bounded[MIN <: Int, MAX <: Int] <: Int = MAX match 
  case MIN => MIN
  case ? => MAX | Bounded[MIN,MAX-1]

val zero15: Bounded[1,5] = 0      // ERROR
val one15: Bounded[1,5] = 1
val two15: Bounded[1,5] = 2
val three15: Bounded[1,5] = 3
val four15: Bounded[1,5] = 4
val five15: Bounded[1,5] = 5
val six15: Bounded[1,5] = 6       // ERROR

// A type for allowed indices, zero-based, for an indexed collection of size N.
type IndexOf[N <: Int] = Bounded[0,N-1]

val zero5: IndexOf[5] = 0
val one5: IndexOf[5] = 1
val two5: IndexOf[5] = 2
val three5: IndexOf[5] = 3
val four5: IndexOf[5] = 4
val five5: IndexOf[5] = 5         // ERROR