def example =
    def a: Int = b
    val b: Int = a
    a

-- [E039] Reference Error: example.scala:2:17 ----------------------------------
2 |    def a: Int = b
  |                 ^
  |       forward reference to b extends over the definition of b (on line 3)
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | b is used before you define it, and the definition of b
  | appears between that use and the definition of b.
  |
  | Forward references are allowed only if there are no value definitions between
  | the reference and the definition that is referred to.
  | Specifically, any statement between the reference and the definition
  | cannot be a variable definition, and if it's a value definition, it must be lazy.
  |
  | Define b before it is used,
  | or move the definition of b so it does not appear between
  | the declaration of b and its use,
  | or define b as lazy.
   -----------------------------------------------------------------------------

// Make the forward dependency lazy
// Warning: It still might fail at runtime
def example =
    def a: Int = b
    lazy val b: Int = a
    a

// Even better ensure to always break the dependecny chain
def example =
  lazy val a: Int = if b == 0 then 1 else b
  lazy val b: Int = 0
  a