"could not inline" warning

Hi,

so I have this seemingly innocent code with an explicit shelf type ;-)

case class Book(id: Int, name: String)

case class Shelf(books: Map[Int, Book]) {
def add(b: Book) = copy(books = books + (b.id -> b))
}

When compiled with "scalac -optimise shelf.scala", there's a warning being printed:

shelf.scala:4: warning: Could not inline required method -> because it is unsafe (target may reference private fields).
def add(b: Book) = books + (b.id -> b)
^
Now, since warnings usually want to tell me that I am doing something can anybody enlighten me what I am doing wrong and what I could do better at that position?

Of course "copy(books = books updated (b.id, b))" gets rid of the warning... but I find that much less expressive.

Any suggestions? (I'm on RC3, btw)

Thanks
Andreas