ListBuffer.++, and List.+

I think this should be made to work:

val x = new ListBuffer[Int]
x += 1
x ++ x

Now it throws an OOME.

Also, when I try this:
val x = List(1)
x += 2

I get a compiler error "reassignment to val".
But there is no "+=" method defined in list, only "+". Does the compiler treat "x += y", when there is no x.+= method, as "x = x + y"?

Thanks,
Dimitris