if a project uses a particular version of scala, but a transitive dependency uses one of the scala-lang optionals (compiler, scalap, reflect, etc) then the older version is used. It would be good if something like this was automatic
dependencyOverrides ++= Set(
"org.scala-lang" % "scala-compiler" % scalaVersion.value,
"org.scala-lang" % "scala-library" % scalaVersion.value,
"org.scala-lang" % "scala-reflect" % scalaVersion.value,
"org.scala-lang" % "scalap" % scalaVersion.value
),