Case class extending trait not working with copy

Question

I try

trait Foo[A] {
  def copy(int: Int): A
}

case class Bar(int: Int) extends Foo[Bar]

but I get

error: class Bar needs to be abstract, since method copy in trait Foo of type (int: Int)this.Bar is not defined

Since Bar is a case class, it automatically defines a copy method with exactly this signature.

Why doesn't the Foo class satisfy the interface defined by the trait Bar?

The case class won't implement copy if it's inherited, even if abstract. There is a duplicate of this somewhere. — Michael Zajac
@m-z, found it! stackoverflow.com/questions/17937020/… That is itself marked as a duplicate, IMO incorrectly. — Paul Draper

ialekseev ialekseev · Accepted Answer · 2015-05-18T08:51:06

I am quoting the Scala's specification:

A method named copy is implicitly added to every case class unless the class already has a member (directly defined or inherited) with that name, or the class has a repeated parameter.

Case class extending trait not working with copy

1 Answers