The following:

    case class Foo(x: Int) extends AnyVal {
        def square: Int = x * x
    }
    val bar: Foo = Foo(5)
    println(bar.square) // => 25

Compiles down to (the moral equivalent of)

    object Foo {
        def square(x: Int): Int = x * x
    }
    val bar: Int = 5
    println(Foo.square(bar)) // => 25

If the inner value (x, in this case) can be stack-allocated or stored in registers then `Foo(x): Foo` will as well. `Foo(x): Bar` (where Bar is a supertype of Foo) will be autoboxed, but that also applies to primitive types (`5: Any` becomes a java.lang.Integer in memory).