Scala lazy val interpretation

I am learning Scala basics. I just came across the lazy val concept. I have following code snippets which work without errors/warnings

Case 1

lazy val a = 10 + b
lazy val b = 5
println(a)

Case 2

lazy val a = 10 + b
val b = 5
println(a)

Case 3

val a = 10 + b
lazy val b = 5
println(a)

I understand how case 1 & 2 work. But I don't understand how the code in case 3 is working without error/warning. How is Scala able to evaluate the value of a when b is not yet defined?

EDIT

I am not running this code in Scala REPL. I have saved the code in case 3 in file called lazyVal.scala. I am executing it using scala lazyVal.scala. I think scala interprets the code in the file.

If I change the code in lazyVal.scala to

val a = 10 + b
val b = 5
println(a)

And execute it using scala lazyVal.scala I do get warning

/Users/varun.risbud/scalaRepo/src/Chapter1/lazyVal.scala:1: warning: Reference to uninitialized value b
val a = 10 + b
         ^
one warning found
10

Also If I change the code to create object and extend App it works

object lazyVal extends App {
    val a = 10 + b
    lazy val b = 5
    println(a)
}
➜  Chapter1 scalac lazyVal.scala
➜  Chapter1 scala lazyVal
15

My scala version is 2.12.1 if that makes any difference.