Missing "Reference to uninitialized value" warning in traits

[lrytz]

reproduction steps

scala> class C { val x = y; val y = 1 }
                         ^
       warning: Reference to uninitialized value y
class C

scala> trait T { val x = y; val y = 1 }
trait T

problem

The warning should also be emitted in the second example.

[seratch]

I explored ways to fix this issue a bit but I found that it's not so simple. scala.tools.nsc.transform.Constructors has a Set of uninitialized values (uninitializedVals). However, in the case of trait initialization, the process to generate the set is not called. Here are some debug print results: https://gist.github.com/seratch/f395371436ce8ec936f3ca4dd07dd893

I hope this is somewhat helpful for other contributors.