问为什么这个简单的例子不能编译？

EN

abstract class Box[+A] {
    var value: A = _
}

covariant type A occurs in contravariant position in type A of parameter of setter value_=

class StringBox extends Box[String]
class DateBox extends Box[Date]

object Testbox {
    def main(args: Array[String]) {
        val list = ListBuffer[Box[Any]]()
        val str = new StringBox
        str.value = "abc"
        val date = new DateBox
        date.value = new Date
        list += str
        list += date
        println(list)
    }
}