我想创建一个通用的总结方法，使用数字，字符和字符串

fun <T> sumAndPrint(vararg elements: T?): T where T : Char?, T : String?, T : Number? {
        when (elements[0]) {
            is String, is Char -> {
                var result: String = ""
                elements.forEach { result += it.toString() }
                return result as T
            }

            is Number -> {
                var result: Double = 0.0
                elements.forEach { result += it.toDouble() }
                return when(elements[0]){
                    is Float, is Double -> result as T
                    is Int -> result.toInt() as T
                    is Byte -> result.toInt().toByte() as T
                    else -> result as T
                }
            }

            else -> {
                println("Unsupported type")
                return null
            }
        }
    }

字符串
但是我遇到了几个错误，比如“只有一个上界可以是一个类”。
我试过不同的方法，但我没有找到一个解决办法，我只是想知道这样的事情是否可能