types.scala

class Foo { def hi = "Hello from Foo!" }
class Bar extends Foo { override def hi = "Hello from Bar!" }

abstract class Consumer {
  type R <: Foo // abstract type R that is Foo or extends Foo
  def greet(foo: R) = foo.hi // method "greet" with 1 arg "foo" of type R, calls and returns foo.hi
}

class FooConsumer extends Consumer { 
  type R = Foo // Concretize type R to Foo.
}

class BarConsumer extends Consumer {
  type R = Bar // Only accept Bar, not Foo.
}

// Works.
println(new FooConsumer().greet(new Foo))
println(new FooConsumer().greet(new Bar))
println(new BarConsumer().greet(new Bar))
// Does not compile.
// /home/arturas/types.scala:22: error: type mismatch;
//  found   : this.Foo
//  required: this.Bar
println(new BarConsumer().greet(new Foo))