warn unused regression involving refutability check in for comprehension

[adriaanm]

Looks like scala/scala#6190 regressed linting for unused stuff a local val captured in a CBN closure is not seen as used:

class Test {
  def timed[T](body: => T) = body

  def run(): Boolean = {
    val grouped = List(("bla","bla"))


    val (_, _) = timed {
      for ((kind, paths) <- grouped) {
        println("")
      }
      (1, 2)
    }
    
    false
  }
}

nok:

➜  Desktop sbt 'set resolvers += "pr-integration" at "https://scala-ci.typesafe.com/artifactory/scala-integration"' 'set scalaVersion := "2.12.5-bin-56918e4"' 'set scalacOptions ++= Seq("-Xfatal-warnings", "-Xlint:unused")' compile
[info] Compiling 1 Scala source to /Users/adriaan/Desktop/target/scala-2.12/classes...
[error] /Users/adriaan/Desktop/unused_captured.scala:5: local val grouped in method run is never used
[error]     val grouped = List(("bla","bla"))
[error]         ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed Mar 8, 2018 11:25:25 AM

ok:

➜  Desktop sbt 'set resolvers += "pr-integration" at "https://scala-ci.typesafe.com/artifactory/scala-integration"' 'set scalaVersion := "2.12.5-bin-f78f517"' 'set scalacOptions ++= Seq("-Xfatal-warnings", "-Xlint:unused")' compile
[info] Compiling 1 Scala source to /Users/adriaan/Desktop/target/scala-2.12/classes...
[success] Total time: 3 s, completed Mar 8, 2018 11:25:49 AM

[som-snytt]

Can I get a Scala/Center t-shirt with stripes in blocker red for Scala and regression orange for Center?

[som-snytt]

@adriaanm There's also a weird error in which it refuses to create /tmp/Test.class. This is fresh 2.12.x on MacOS.

$ scalac -d /tmp -Xlint  oops.scala
oops.scala:3: warning: type parameter T defined in method timed shadows trait T defined in package <empty>. You may want to rename your type parameter, or possibly remove it.
  def timed[T](body: => T) = body
            ^
one warning found
$ skalac -d /tmp -Xlint  oops.scala
oops.scala:3: warning: type parameter T defined in method timed shadows trait T defined in package <empty>. You may want to rename your type parameter, or possibly remove it.
  def timed[T](body: => T) = body
            ^
oops.scala:6: warning: local val grouped in method run is never used
    val grouped = List(("bla","bla"))
        ^
error: error writing Test: Can't create directory /tmp; there is an existing (non-directory) file in its path
two warnings found
one error found

[adriaanm]

Smaller version (fits on a t-shirt sleeve):

class Test {
  def xsUnused = {
    val xs: List[Int] = List(0)

    for (refute@1 <- xs) {} // xs is reported as unused when it's hidden in a synthetic withFilter call?
  }
}

[lrytz]

scala/scala#6404