Unsound Kotlin

unsound.kt

object Test {

  @JvmStatic
  fun main(args: Array<String>) {
    val strs = supplier("foo", "bar")
    val ints = supplier(1, 2)
    mergeSuppliers(strs, ints)
    val s = strs()[2] // boom!
  }

  private fun mergeSuppliers(vararg suppliers: () -> MutableList<*>) =
    mergeLists(suppliers.map { it() }) // shouldn't type-check, but does

  private fun<T> mergeLists(lists: List<MutableList<T>>) {
    if (lists.size >= 2)
      lists[0].addAll(lists[1])
  }

  private fun<T> supplier(vararg xs : T): () -> MutableList<T> =
    xs.toMutableList().let { l -> { l }}
}