Final vals in traits should be disallowed

[scabug]

Marking a val declared in a trait final does not actually cause the emitted field to be final, due to the way traits are initialised. Therefore the atomicity guarantee of final does not hold. Additionally, the compiler does not permit vals defined in traits to be overridden anyway, so it appears that the final keyword accomplishes nothing at all in this case except to mislead programmers and cause subtle bugs.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-201?orig=1
Reporter: Jamie Webb (jwebb)

[scabug]

@odersky said:
It's not so simple -- marking the `val' final means that it might treated as a constant value:

e.g.

trait T {
final val x = 0 // x is treated as an alias for the Literal `0'.
}

Dropping the final would change that.

[scabug]

@odersky said:
Maybe all values should be final fields? I leave for Iulian to investigate.

[scabug]

@dragos said:
Made all values to be final fields.

[scabug]

@odersky said:
Milestone 2.7.0 deleted