Restricting a trait to objects?

Question

Is there a way to restrict a trait so that it can only be mixed into objects? E.g.

trait OnlyForObjects {
  this: ... =>
}

object Foo extends OnlyForObjects  // --> OK

class Bar extends OnlyForObjects   // --> compile error

Answer 1

Yes! There's the obscure and mostly undocumented scala.Singleton:

scala> trait OnlyForObjects { this: Singleton => }
defined trait OnlyForObjects

scala> object Foo extends OnlyForObjects
defined module Foo

scala> class Bar extends OnlyForObjects
<console>:15: error: illegal inheritance;
 self-type Bar does not conform to OnlyForObjects's selftype OnlyForObjects
 with Singleton
       class Bar extends OnlyForObjects
                         ^

It's mentioned a few times in the language specification, but doesn't even appear in the API documentation.