What is the difference between self-types and trait subclasses?

Question

A self-type for a trait A:

trait B
trait A { this: B => }

says that \"A cannot be mixed into a concrete cl

Answer 1

Update: A principal difference is that self-types can depend on multiple classes (I admit that's a bit corner case). For example, you can have

class Person {
  //...
  def name: String = "...";
}

class Expense {
  def cost: Int = 123;
}

trait Employee {
  this: Person with Expense =>
  // ...

  def roomNo: Int;

  def officeLabel: String = name + "/" + roomNo;
}

This allows to add the Employee mixin just to anything that is a subclass of Person and Expense. Of course, this is only meaningful if Expense extends Person or vice versa. The point is that using self-types Employee can be independent of the hierarchy of the classes it depends on. It doesn't care of what extends what - If you switch the hierarchy of Expense vs Person, you don't have to modify Employee.