关于phantom type的一个简陋的kotlin的例子，作用如main所示

PhantomType.kt

fun main(): Unit = with(SomeBuilder) {
   SomeBuilder.getBuilder().setS2("2").setS1("1").buildSome() // ok
   SomeBuilder.getBuilder().setS1("1").setS2("2").buildSome() // ok
   SomeBuilder.getBuilder().setS1("1").buildSome() // error
   SomeBuilder.getBuilder().setS2("2").buildSome() // error
   SomeBuilder.getBuilder().buildSome() // もerror
}

class Some(val s1: String, val s2: String)

class SomeBuilder<T : State, U : State> private constructor() {
   private lateinit var s1: String
   private lateinit var s2: String

   companion object {
      fun getBuilder() = SomeBuilder<NotReady, NotReady>()
      fun <T : State, U : State> SomeBuilder<T, U>.setS1(s1: String) = this.apply { this.s1 = s1 } as SomeBuilder<S1Ready, U>
      fun <T : State, U : State> SomeBuilder<T, U>.setS2(s2: String) = this.apply { this.s2 = s2 } as SomeBuilder<T, S2Ready>
      fun SomeBuilder<S1Ready, S2Ready>.buildSome() = Some(this.s1, this.s2)
   }
}

sealed class State
object S1Ready : State()
object S2Ready : State()
object NotReady : State()

萌雀巨佬赛高！