Why implicit class in Scala must reside in another trait/class/object?

Question

Based on the scala documentation: http://docs.scala-lang.org/overviews/core/implicit-classes.html, the implicit class has three restrictions, and the very first one, I quote here, is

They must be defined inside another trait/class/object

What is the intuition/reason to explain/justify such a restriction?

Answer 1

An implicit class decomposes into a "normal" class and an implicit method that instantiates the class:

implicit class IntOps(i: Int) { def squared = i * i }

Is rewritten as

class IntOps(i: Int) { def squared = i * i }
implicit def IntOps(i: Int) = new IntOps(i)

But in Scala, you cannot define a method (def IntOps) outside of an object or a class. That is why.