How to return subtype of a generic trait in scala?

Question

I am trying to create a factory pattern, and the return type is a parametrized trait, and the actual return type will be a subtype of that trait, but I do not know the generic type beforehand, so I don't know how to specify the return type. A simplified version is like this:

trait Mapper[T] {
    def createMap(a: T, b: T): T
}

class MapperA extends Mapper[String]{
    override def createMap(a: String, b: String): String = {
        a + b
    }
}

class MapperB extends Mapper[Int]{
    override def createMap(a: Int, b: Int): Int = {
        a + b + b + b
    }
}

def factory(typeOfMapper: String): Mapper = {
    typeOfMapper match {
        case "mapperA" => new MapperA()
        case "mapperB" => new MapperB()
    }
}

val mapper = factory("mapperA")

This will give me and error of

trait Mapper takes type parameters

but I do not know the generic type beforehand. What should the return type be for the factory method here?