Observations

class Test extends AnyFlatSpec with Matchers {
"transformer" should "work" in {
    // source: https://contributors.scala-lang.org/t/ability-to-force-the-caller-to-specify-a-type-parameter-for-a-polymorphic-method/2116/26
    sealed trait NotNothing[-T]
    
    object NotNothing {
      @implicitAmbiguous("inst() method needs a generic parameter type, which is missing")
      implicit val nothingIsNothing = new NotNothing[Nothing]{}
      implicit def notNothing[T] = new NotNothing[T] {}
    }
    
    implicit class Transformer(source: String) {
      def as[T : NotNothing : ClassTag] = {
        import scala.reflect._
        val cls: Class[T] = classTag[T].runtimeClass.asInstanceOf[Class[T]]
        val cons: Constructor[T] = cls.getDeclaredConstructor(classOf[String])
        cons.newInstance(source)
      }
    }

    val a0 = "good A".as[A]
    val b0 = "good B".as[B]
    a0.getClass shouldBe classOf[A]
    b0.getClass shouldBe classOf[B]
    a0.toString shouldBe "A(good A)"
    b0.toString shouldBe "B(good B)"
  }
}

class Base
case class A(val s: String) extends Base
case class B(val s: String) extends Base