list.filter(!=) compiles, but doesn't work as expected

Question

The following Scala code compiles but does not do what I expected:

scala> List((1,1),(1,2)).filter(!=)
res1: List[(Int, Int)] = List((1,1), (1,2))

What does != refer to in above code?

I know that I can write the predicate correctly as

scala> List((1,1),(1,2)).filter { case (a, b) => a != b }
res1: List[(Int, Int)] = List((1,2))

but I'm curious what the first expression actually does.

Answer 1

The Scala compiler does the following expansion (you can see this yourself when running scalac with the -Xprint:typer flag):

List.apply[(Int, Int)]
      (scala.Tuple2.apply[Int, Int](1, 1), scala.Tuple2.apply[Int, Int](1, 2))
    .filter(((x$1: Any) => this.!=(x$1)));

Meaning it attempts to compare this agains't your tuple which is lifted to Any, which is not what you're trying to do.