arg是number类型 那arg+1肯定是number类型。函数返回值Type 也对吧？

arg是number类型 那arg+1肯定是number类型。函数返回值Type 也对吧。为什么报错呢

function identity<Type>(arg: Type): Type {
  if(typeof arg === 'number' ){
    //Type 'number' is not assignable to type 'Type'.
    // 'Type' could be instantiated with an arbitrary 
    //type which could be unrelated to 'number'.
    return arg + 1  
  }else {
    return arg
  }
}