Ok I see now. It was working on Scala 2.11 but not on higher versions, because then those two implicits are ambiguous. Because Source is also AutoCloseable. You solve that by assigning lower priority to one of those implicits.
trait LowerPriority {
implicit object AutoCloseableIsReleasable extends Releasable[AutoCloseable] {
def release(resource: AutoCloseable): Unit = resource.close()
}
}
object Releasable extends LowerPriority {
implicit object SourceIsReleasable extends Releasable[Source] {
def release(resource: Source): Unit = resource.close()
}
}
Probably a def with a type parameter automatically has lower priority and that’s why the other solution also worked.