Scala warning erasure in case

Question

I have the following pattern matching case in a scala function:

def someFunction(sequences: Iterable[Seq[Int]]):Seq[Int] = sequences match{
    case Seq() => Seq(1)
    case _ => ...
    ...
}

And I get the following warning:

warning: non variable type-argument A in type pattern Seq[A] is unchecked since it is eliminated by erasure
case Seq(_) => Seq(1)
        ^
one warning found

What does this mean?

Answer 1

This warning is a bit spurious, and will not be present on Scala 2.10. In fact, I think it's a regression from Scala 2.8 (that is, it is not present there).

The reason for the warning is that it interprets Seq(_) to mean Seq(_: Seq[Int]), since that's the type parameter of sequences, and then complaining that it can't guarantee that Int there, since, at compile time, that will be erased. As I said, it's spurious.