Scala: Why does Seq.contains take an Any argument, instead of an argument of the sequence type?

So for example why does List(1,2,3,4).contains("wtf") even compile? Wouldn't it be nice if the compiler rejected this?

Lots of interesting answers, but here's my own theory: if contains did not receive an Any, then Seq could not be co-variant.

See, for instance, Set, which is not co-variant and whose contains take an A instead of an Any.

The reasons for that is left as an exercise to the reader. ;-) But here is a hint:

scala> class Container[+A](elements: A*) {                         
     |   def contains(what: A): Boolean = elements exists (what ==)
     | }
<console>:7: error: covariant type A occurs in contravariant position in type A of value what
         def contains(what: A): Boolean = elements exists (what ==)
                      ^

"contains" is fundamentally about equality testing, and equality in Scala (as in Java before it) is untyped. The practical value of having untyped-equality is small, but not zero. There are, for instance, a few occasions where it makes sense for two objects of different classes to be equal to one another. For instance, you might wish an object of type RGBColor to be equal to a PantoneColor if they define the same hue, or an immutable HashSet and an immutable TreeSet to be equal if they contain the same elements. That said, untyped-equality also causes a bunch of headaches, and the fact that the compiler could easily catch that List(1,2,3,4).contains("wtf") is nonsensical but won't is one of them.

Most Java bug-finding tools include tests to detect the presence of improbable untyped-equality uses. (I wrote the inspections to do this in IntelliJ IDEA.) I have no doubt that when Scala bug-finding tools come online, these will be among the first bugs detected.

SeqLike.contains checks whether a value is present by checking for an element in the sequence that is equal to the value (using ==). == takes an Any so I suspect that this is the reason.