One handy feature of Scala is lazy val, where the evaluation of a val is delayed until it's necessary (at first access).

Ofcourse a lazy val must have some overhead - somewhere Scala must keep track of whether the value has already been evaluated and the evaluation must be synchronized, because multiple threads might try to access the value for the first time at the same time.

What exactly is the cost of a lazy val - is there a hidden boolean flag associated with a lazy val to keep track if it has been evaluated or not, what exactly is synchronized and are there any more costs?

And a follow-up question: Suppose I do this:

class Something {
    lazy val (x, y) = { ... }
}

Is this the same as having two separate lazy vals x and y or do I get the overhead only once, for the pair (x, y)?