scala> case class Example[T](foo: T) { def string[T <: String](): String = s"$foo" }
defined class Example

scala> val f = Example("bar")
f: Example[String] = Example(bar)

scala> f.string
res0: String = bar

scala> val b = Example(1)
b: Example[Int] = Example(1)

scala> b.string
res1: String = 1

scala>