Scala indexOf accepts everything

Question

I wrote this into a REPL:

case class Thingy(s: String)
val things = List(Thingy("x"), Thingy("y"))
things.indexOf("x")

That returns -1. But I would have expected it not to compile, because "x" is a String, not a Thingy. In fact, it turns out it doesn't matter what type you put into indexOf:

things.indexOf(42)
things.indexOf(java.time.LocalDate.now())

These all return -1.

indexOf has this signature:

def indexOf[B >: A](elem: B): Int

I thought >: means B should be a supertype of A, but none of these classes are a supertype of my case class Thingy.

What am I missing here?

Answer 1

B is inferred as java.io.Serializable, which is a supertype of both String and Thingy.¹

This is an unfortunate consequence of two things:

1. Scala's superfluous parent types that all/most values share (Any, Object, Serializable, et al).
2. List being covariant.²

To define indexOf as

def indexOf(elem: A): Int

would put A in a contravariant position, which isn't allowed³ because it would violate the Liskov substitution principle: A List[Thingy] is a List[Any], and you can call .indexOf("x") on a List[Any], therefore you should be able to call .indexOf("x") on a List[Thingy].

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¹ If they didn't both happen to implement Serializable, it would still infer Any.

² This is a reason to prefer invariant collections, such as scalaz.IList.

³ Try it - it won't compile: trait Foo[+A] { def bar(a: A) }