This is a famous or “infamous” issue.
It was not an easy search for me, so for the record:
If anyone is interested, I found the link by adding “tpolecat” to the search, as I remembered that abstract case class is a tpolecatism, or tpolecatechism I guess is the right word.
In Scala 2, unless otherwise constrained, always use -Xsource:3 to achieve further hints about whether your code will compile the same (we abjure here any petty notion of correctness).