请教scala的隐式转换问题?报错

一个自定义的类Rational，想让这个类跟原生的类型int之间实现隐式转换，实现四则运算

class Rational(n:Int,d:Int) {
 
 private val number = n
 private val denom = d
 println("create->"+n+"/"+d)
 
 require(d != 0)

 def this(n:Int) {
 this(n,1)
 }

 def +(that: Rational): Rational = {
 new Rational( number * that.denom + that.number * denom, denom * that.denom )
 }
 
 def +(i: Int): Rational = {
 new Rational(number + i * denom, denom)
 }

 implicit def intToRational(x: Int):Rational = {
 new Rational(x)
 }
}

object Test {
	def main(args : Array[String]) {
              var x1 = new Rational(1,2)
		println(2 + x1)
		
	}
}

println(2 + x1)

这段还是会报错，虽然在自定义类中加了隐式转换函数但还是有问题