Scala: implementing method with return type of concrete instance

Question

I need a way to enforce a method in an abstract class to have a return type of the concrete class of the object it is called on. The most common example is a copy

Answer 1

Do not force the type bound on the declaration side, unless you need that bound within the definition of A itelf. The following is sufficient:

abstract class A(id: Int) {
  type Self
  def copy(newId: Int): Self
}

Now force the Self type on the use site:

def genNewId(): Int = ???
def createCopies[A1 <: A { type Self = A1 }](seq: Seq[A1]): Seq[A1] = 
  seq.map(_.copy(genNewId()))