Pretty please support sub-projects with different Scala versions.

[steinybot]

My use case for needing sub-projects with different Scala versions is when there are 2 sub-projects, one that is a Scala library that is cross built for Scala 2.12 and 2.13 and another sub-project which is an sbt plugin that can only be built with Scala 2.12.

[steinybot]

I may have found a workaround:

lazy val sbtProject = (project in file("sbt"))
  .dependsOn(core)
  .enablePlugins(SbtPlugin)
  .settings(commonProjectSettings)
  .settings(
    skip := {
      CrossVersion.partialVersion((core / scalaVersion).value) match {
        case Some((2, n)) if n == 13 => true
        case _ => false
      }
    },
    crossScalaVersions := Nil,
  )

[steinybot]

Well that didn't work.

Latest iteration is:

import explicitdeps.ExplicitDepsPlugin
import explicitdeps.ExplicitDepsPlugin.autoImport._
import sbt.Keys._
import sbt._
import sbt.plugins.SbtPlugin

// This is all the crazy hacks to get cross compiling working with an sub-project that is an sbt plugin.
object SbtSubProjectPluginPlugin extends AutoPlugin {

  override def trigger: PluginTrigger = allRequirements
  override def requires: Plugins      = ExplicitDepsPlugin && SbtPlugin

  private val sspppIsScala213 = settingKey[Boolean]("Checks if the current Scala version is 2.13")

  override def projectSettings: Seq[Def.Setting[_]] =
    List(
      crossScalaVersions := Nil,
      libraryDependencies := libraryDependenciesSetting.value,
      projectDependencies := projectDependenciesTask.value,
      sspppIsScala213 := {
        if (isScala213(scalaVersion.value))
          throw new IllegalStateException("sbt project must not use Scala 2.13. Did you force the version with '+'?")
        isScala213Setting.value
      },
      // We can't skip this as it has to run at least once or sbt complains.
      update / skip := false,
      // Skip everything else otherwise it will just fail.
      skip := sspppIsScala213.value,
      undeclaredCompileDependenciesFilter -= moduleFilter()
    )

  private def isScala213Setting = Def.setting {
    val versions =
      scalaVersion.all(ScopeFilter(inDependencies(ThisProject, transitive = true, includeRoot = false))).value
    versions.exists(isScala213)
  }

  private def isScala213(version: String) =
    CrossVersion.partialVersion(version) match {
      case Some((2, n)) if n == 13 => true
      case _                       => false
    }

  private def projectDependenciesTask = Def.task {
    // Remove all project dependencies for Scala 2.13 as they will not resolve when cross building.
    if (sspppIsScala213.value) {
      Seq.empty
    } else {
      projectDependencies.value
    }
  }

  private def libraryDependenciesSetting = Def.setting {
    // Remove all library dependencies for Scala 2.13 as they will not resolve when cross building.
    if (sspppIsScala213.value) {
      Seq.empty
    } else {
      libraryDependencies.value
    }
  }
}

[kubukoz]

@steinybot just out of curiosity, what would happen if you set crossScalaVersions in ThisBuild to the whole set of versions, but with specific overrides in each project?

[steinybot]

That doesn't seem to make a difference. For the record the problem I have actually has nothing to do with sbt-github-actions instead it is sbt/sbt#5586.

[djspiewak]

Yeah this is a serious limitation of sbt right now. There are a lot of crazy hacks to get around it but none of them work well. It's also something that you somewhat-forcibly hit whenever you have Scala Native in your build.