error on obvious forward ref

[adriaanm]

From scala-language

On Tuesday, March 22, 2011 at 5:14:16 PM UTC+2, Eugen Labun wrote:
Hi all,
eventually (or even surely) this is an old question but I haven't found an explanation for this
simple situation (nor in Language Specification, nor in Scala-Book).

(More complicated cases are described in this excellent faq from Paul:
https://github.com/paulp/scala-faq/wiki/Initialization-Order)

Constructions like

  def m {
    val x = y // compile error "forward reference extends over definition of value x"
    val y = 5
  }

cause a compile error if used in methods,
but are accepted in constructors/initializers:

  class/object T {
    val x = y // compiles OK, x = 0
    val y = 5
  }

The 'x' in the code above gets initialized to 0 (!), not to 5.

Both -- acceptance by the compiler and initialization to 0 -- are counterintuitive to me.
Would a compile error for the second case ("illegal forward reference") not be a more preferable?

[adriaanm]

This already produces a warning:

Welcome to Scala 2.12.0-RC1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions for evaluation. Or try :help.

scala> class T {
     |   val x = y // compiles OK, x = 0
     |   val y = 5
     | }
<console>:12: warning: Reference to uninitialized value y
         val x = y // compiles OK, x = 0
                 ^
defined class T

[adriaanm]

Not sure we should do more.