2.13 regression in @tailrec + singleton types

[SethTisue]

at e.g. https://scala-ci.typesafe.com/view/scala-2.13.x/job/scala-2.13.x-integrate-community-build/919/consoleFull we see:

[akka-actor] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/akka-actor-75f78e641cd66395240926f2b01f241f7dee1dce/akka-actor/src/main/scala/akka/util/ByteIterator.scala:277:33: could not optimize @tailrec annotated method dropWhile: it has a singleton result type which cannot be erased correctly
[akka-actor] [error]     @tailrec final override def dropWhile(p: Byte ⇒ Boolean): this.type =
[akka-actor] [error]                                 ^
[akka-actor] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.9/project-builds/akka-actor-75f78e641cd66395240926f2b01f241f7dee1dce/akka-actor/src/main/scala/akka/util/ByteIterator.scala:312:34: could not optimize @tailrec annotated method getToArray: it has a singleton result type which cannot be erased correctly
[akka-actor] [error]     @tailrec protected final def getToArray[A](xs: Array[A], offset: Int, n: Int, elemSize: Int)(getSingle: ⇒ A)(getMult: (Array[A], Int, Int) ⇒ Unit): this.type =
[akka-actor] [error]                                  ^
[akka-actor] [warn] 109 warnings found
[akka-actor] [error] two errors found

@milessabin since it involves singleton types, I'm tentatively assigning this to you for investigation.

[jrudolph]

The change here is not just a nuisance but a regression since, as the error message says, tail call optimization isn't done any more.

[milessabin]

Agreed. I'm on it.

[jrudolph]

Cool, thanks Miles.

[milessabin]

Fixed in scala/scala#6420.