Question

On migrating our code to Scala 2.9 we’ve found large swathes of it that didn’t work and failed silently. We tracked it down to case classes that extend Proxy not being equal. In our code we don’t extend Proxy directly, we just extend classes in libraries that extend Proxy.

Any help would be greatly appreciated.

In 2.8

scala> case class Test(a:String) extends Proxy {
     |   def self = a
     | }
defined class Test

scala> 

scala> val label = new Test("bla")
label: Test = bla

scala> println(label == label) // this is TRUE
true

scala> println(label == "bla")
true

In 2.9

scala> case class Test(a:String) extends Proxy {
     |   def self = a
     | }
defined class Test

scala> 

scala> val label = new Test("bla")
label: Test = bla

scala> println(label == label) // this is now FALSE
false

scala> println(label == "bla")
true

Update

I think this can only be a bug in Scala 2.9. Otherwise if you have a case class that extends any other class you have to investigate that base class’s hierarchy to make sure at no point is it extending Proxy. We won’t be able to do this in our code, we’ll just be able to fix the more obvious bugs. If this is intended behaviour then a compiler warning is a must. Does that sound about right?

Update

Also being discussed on the scala mailing list.

Update

I’ve filed a bug

Answer 1

In 2.9 they changed the equals method from:

override def equals(that: Any): Boolean = 
  if(that == null) false 
  else that equals self

to

override def equals(that: Any): Boolean = that match {
 case null       => false
 case x: Equals  => (x canEqual self) && (x equals self)
 case x          => (x equals self)
}

x: Equals doesn’t equal self for some reason.

You can override the equals method to fix it.