final vals in traits not handled properly in Scala 2.12

[gslowikowski]

The original issue is here: scoverage/sbt-scoverage#203

I investigated, how it works with different Scala versions (2.10.6, 2.11.8 and 2.12.0) on very simple trait:

package test

trait TraitWithFinalVal {
  final val FOO = "Bar"
}

I don't feel qualified to fix this issue, this description is for @sksamuel (or anyone knowing scalac internals).

Scoverage during instrumentation replaces simple constant

"Bar"

with a block:

{
  scoverage.Invoker.invoked(1, "path/to/scoverage-data");
  "Bar"
}

The type of this block should be equal to the type of the last statement ("Bar").
It's set by this typer call:

            val apply = invokeCall(id)
            val block = Block(List(apply), tree)
            localTyper.typed(atPos(tree.pos)(block)) // <------ here

What are the types of uninstrumented and instrumented val in different Scala versions:

Scala version	uninstrumented (simple value) type	instrumented (block) type	difference?	problem?
2.10.6	String	String	no	no
2.11.8	String("Bar")	String	yes	no
2.12.0	String("Bar")	String	yes	YES!

As you can see, after instrumentation the resulting block type is different from val type even with Scala 2.11, but it doesn't lead to compiler error. I found the exact git commit which changed things - it's scala/scala@50462a4

I'm not sure if typed method returns wrong type, or Scoverage should not depend on it and set the type of the block to the type of the constant (something like setType(tree.tpe)).

[sksamuel]

This is fixed now.

[gslowikowski]

Summary of the problem, from my point of view:

There was a bug in Scala 2.12.0 - scala/scala-dev#219.
It's fixed in 2.12.1 (commit scala/scala@71a2cad)
and it fixes our scoverage problem.

Commits: aa3c5ed and 8bc8c22 change too much.

Most importantly this change:

case v: ValDef if v.rhs.isInstanceOf[Literal] => tree

(aa3c5ed#diff-e85576328aa1f74addcb13a5f7546600R585)

E.g. the number of statements in scala library projects changes from 37918 to 37261. This is not acceptable for minor bugfix release.

This change aa3c5ed#diff-e85576328aa1f74addcb13a5f7546600L608 introduces a bug.

I've tested this on Scala project and "updateLocation" should be restored here.
Check for example Specializable class (https://github.com/scala/scala/blob/2.12.x/src/library/scala/Specializable.scala).

All statements after Group class definition have classType = "Class" and fullClassName = "Specializable.Group".
They should have classType = "Object" and fullClassName = "Specializable".

Simplified class to test:

object Specializable {
  trait SpecializedGroup { }

  class Group[T >: Null](value: T) extends SpecializedGroup { }

  final val Primitives  = new Group((Byte, Short, Int, Long, Char, Float, Double, Boolean, Unit))
}

This should be fixed/cleaned because, from my point of view, it blocks further scoverage development.

[gslowikowski]

@sksamuel I'd like to progress with development, but this issue blocks me.