"Reference to uninitialized value" warning not produced for more complicated reference patterns

[cnestor]

Hey guys! Not sure how hard this would be to fix, but I thought I would bring it up.

//produces compiler warning
class C1 {
  val x = y
  val y = 1
}

//no warning :(
class C2 {
  val x = getY
  val y = 1
  def getY = y
}

//no warning either :(
trait T {
  val y: Int
  val x: Int = y
}

class C3 extends T {
  val y = 1
}

This is a really easy thing to do accidentally if you're moving things around and it can lead to a class that throws a NullPointerException whenever it is instantiated.

Related - #221
IMO it would be better to produce an error than a warning in general, especially since classes/strings/collections/misc non-Int values default to null, which seems to run very counter to the spirit of Scala. ¯\_ (ツ) _/¯

[SethTisue]

this has been an area of active discussion over the years — you might look for related tickets in scala/bug. it is a difficult design and implementation problem. I doubt anything further will change in this area in Scala 2. not sure what the current status over in lampepfl/dotty is

[arpit1543]

I have experienced the same Error while compiling the code.
All i did was just moved around the code and error was gone.

[dwijnand]

If you want, this PR in Dotty: scala/scala3#7789 is a proposal for an experimental -Ycheck-init to check that initialisation is safe.