Uncaught Missing token index error since upgrade to scalafmt-core 3.5.0

[shonfeder]

We've been experiencing an uncaught exception in runs of scalafmtCheckAll when trying to upgrade scalafmt to version 3.5.0. I was not able to reproduce using the scalafmt CLI alone, so am reporting the issue to the sbt plugin repo, as instructed in the scalafmt issue template.

Synopsis

We hit the following error:

> scalafmtCheckAll
[info] scalafmt: Checking 1 Scala sources (/home/sf/scratch/scalafmterr)...
[error] stack trace is suppressed; run last Compile / scalafmtCheck for the full output
[error] (Compile / scalafmtCheck) org.scalafmt.sbt.ScalafmtSbtReporter$ScalafmtSbtError: scalafmt: Missing token index [63:64]: `
[error] ` [/home/sf/scratch/scalafmterr/src/main/scala/example/scalafmterr.scala]
[error] Total time: 0 s, completed Apr 11, 2022, 1:35:10 PM

With the following configs:

.scalafmt.conf:

version = "3.5.1"
runner.dialect = scala212
preset = Scala.js

build.sbt:

ThisBuild / scalaVersion := "2.12.15"

package/plugins.sbt:

addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.6")

With the source file ./src/main/scala/example/scalafmterr.scala with the following content:

package example

trait ExampleTrait {

  def foo(a: Int, b: Int)
  def bar(a: Int)
}

Full reproduction example

See https://github.com/shonfeder/scalafmt-missing-token-err-mwe

References

You can see these failures in the context of our CLI here:

• https://github.com/informalsystems/apalache/runs/5861929467?check_suite_focus=true#step:4:39

Workaround

A workaround is just to annotate the methods' return types:

package example

trait ExampleTrait {

  def foo(a: Int, b: Int): Unit
  def bar(a: Int): Unit
}

[shonfeder]

Thanks for the quick fix!