crossproject-build.sbt

lazy val `core` = crossProject(JSPlatform, JVMPlatform)
  .crossType(CrossType.Full)
  .in(file(".")).
  settings((FMPublic ++ setCrossDirs(Compile) ++ setCrossDirs(Test) ++ Seq( // Note: FMPublic needs to be here for sbt-release to work
...


// Adds a `src/main/scala-2.13+` source directory for Scala 2.13 and newer
// and a `src/main/scala-2.12-` source directory for Scala version older than 2.13
def setCrossDirs(config: Configuration): Seq[Setting[_]] = {
  Seq(
    unmanagedSourceDirectories in config := {
      val baseDir   = baseDirectory.value
      val platform  = crossProjectPlatform.value.identifier

      val configPath: String = config match {
        case Compile => "main"
        case Test    => "test"
        case _       => return Nil
      }

      val javaSources = if (platform != "js") Seq(
        baseDir / "src" / configPath / "java",
      ) else Nil

      javaSources ++ (CrossVersion.partialVersion(scalaVersion.value) match {
        case Some((2, n)) if n < 13 => Seq(
          baseDir                    / "src" / configPath / s"scala-2.$n",
          baseDir / ".."  / "shared" / "src" / configPath / s"scala-2.$n",
          baseDir                    / "src" / configPath / "scala-2.12-",
          baseDir / ".."  / "shared" / "src" / configPath / "scala-2.12-",
          baseDir                    / "src" / configPath / "scala",
          baseDir / ".."  / "shared" / "src" / configPath / "scala",
        )
        case Some((m, n))           => Seq(
          baseDir                   / "src" / configPath / s"scala-$m.$n",
          baseDir / ".." / "shared" / "src" / configPath / s"scala-$m.$n",
          baseDir                   / "src" / configPath / "scala-2.13+",
          baseDir / ".." / "shared" / "src" / configPath / "scala-2.13+",
          baseDir                   / "src" / configPath / "scala",
          baseDir / ".." / "shared" / "src" / configPath / "scala",
        )
      })
    }
  )
}