open class Root<T> {
    val target = SimpleObjectProperty<T>()
    val otherData = SimpleStringProperty()
}

class Strung : Root<String>() {
    val moreOtherData = SimpleStringProperty()
}

class Trail : Root<Path>() {
    val moreDifferentOtherData = SimpleStringProperty()
}

open class RootModel<T, R : Root<T>> : ItemViewModel<R>() {
    val target = bind { item?.target }  // This is the only failing call
    val otherData = bind { item?.otherData }
}

class StrungModel : RootModel<String, Strung>() {
    val moreOtherData = bind { item?.moreOtherData }
}

class TrailModel : RootModel<Path, Trail>() {
    val moreDifferentOtherData = bind { item?.moreDifferentOtherData }
}