What Scala feature allows the plus operator to be used on Any?

I'm still learning Scala, and when I ran across an example in the Koans, I wasn't able to understand why it works:

var foo : An开发者_StackOverflowy = "foo"
println(foo + "bar")

Any doesn't have a + method

There is an implicit conversion in the scala.Predef object:

implicit def any2stringadd(x: Any): StringAdd

StringAdd defines a + operator/method:

def +(other: String) = String.valueOf(self) + other

Also, since scala.Predef is always in scope, that implicit conversion will always work.

It works because of implicit conversions which "fixes" certain type errors for which conversions have been provided. Here is more info on the mechanism of implicit conversions:

http://www.artima.com/pins1ed/implicit-conversions-and-parameters.html#21.2

In fact it uses this very same example x + y to explain how it works. This is from the 1st edition of the book, but the explanation is still valid.