Scala 3 GADTs

Gadt.sc

 object Main {   
    enum Expr[+T] {
      case MyInt(i: Int) extends Expr[Int]
      case Bool(b: Boolean) extends Expr[Boolean]
      case Add(x: Expr[Int], b: Expr[Int]) extends Expr[Int]
      case Eq(x: Expr[T], b: Expr[T]) extends Expr[Boolean]
    }

    import Expr._

    def eval[T](e: Expr[T]): T = {
      e match {
        case MyInt(i) => i
        case Bool(b) => b
        case Add(x, y) => eval(x) + eval(y)
        case Eq(x, y) => eval(x) == eval(y)
      }
    }
    
    def main(args: Array[String]): Unit = {
      println(eval(Add(MyInt(1), MyInt(2))))
      println(eval(Add(Bool(true), MyInt(2)))) // Fails at compile time
    }
 }