Workaround for Intellij "Open cross-compiled projects as Scala 2" option pending upstream merge of JetBrains/sbt-structure#73

Author: neko-kai

build.sbt

@@ -11,6 +11,45 @@ enablePlugins(SbtgenVerificationPlugin)
 
 disablePlugins(AssemblyPlugin)
 
+Global / Keys.commands ~= {
+  // Workaround for Intellij "Open cross-compiled projects as Scala 2" option pending upstream merge of https://github.com/JetBrains/sbt-structure/pull/73
+  xs =>
+    def numbersOf(version: String): (Int, Int, Int) = {
+      val prefix = version.split('.').filter(s => s.nonEmpty && s.forall(_.isDigit)).map(_.toInt).take(3)
+      val xs = prefix ++ Seq.fill(3 - prefix.length)(0)
+      (xs(0), xs(1), xs(2))
+    }
+
+    val preferScala2: Command = Command.command("preferScala2") { state =>
+      val (structure, data) = {
+        val extracted = Project.extract(state)
+        (extracted.structure, extracted.structure.data)
+      }
+
+      val scala3Projects = structure.allProjectRefs.filter { project =>
+        (project / Keys.scalaVersion).get(data).exists(_.startsWith("3."))
+      }
+
+      val (scala2Versions, crossScala2And3ProjectsCount) = scala3Projects.foldLeft((Set.empty[String], 0)) {
+        case ((scala2Versions, crossCount), project) =>
+          val projectScala2Versions = (project / Keys.crossScalaVersions).get(data).getOrElse(Seq.empty).filter(_.startsWith("2."))
+          (scala2Versions ++ projectScala2Versions, crossCount + (if (projectScala2Versions.nonEmpty) 1 else 0))
+      }
+
+      // We can only do this when all sbt projects cross-compile to Scala 2 & Scala 3
+      // See https://youtrack.jetbrains.com/issue/SCL-22619/
+      val canSetScala2VersionGlobally = crossScala2And3ProjectsCount > 0 &&
+        scala3Projects.length == crossScala2And3ProjectsCount
+      if (canSetScala2VersionGlobally) {
+        "++" + scala2Versions.maxBy(numbersOf) :: state
+      } else {
+        state
+      }
+    }
+
+    xs.filterNot(_.nameOption.contains("preferScala2")) :+ preferScala2
+}
+
 lazy val `fundamentals-basics` = crossProject(JVMPlatform, JSPlatform).crossType(CrossType.Pure).in(file("fundamentals/fundamentals-basics"))
   .settings(
     libraryDependencies ++= Seq(

project/Deps.sc

@@ -236,7 +236,48 @@ object Izumi {
 
       final val crossScalaSources = Defaults.CrossScalaPlusSources
 
-      final val topLevelSettings = Seq()
+      final val topLevelSettings = Seq(
+        "Keys.commands".in(
+          SettingScope.Raw("Global")
+        ) ~= """{
+               |  // Workaround for Intellij "Open cross-compiled projects as Scala 2" option pending upstream merge of https://github.com/JetBrains/sbt-structure/pull/73
+               |  xs =>
+               |    def numbersOf(version: String): (Int, Int, Int) = {
+               |      val prefix = version.split('.').filter(s => s.nonEmpty && s.forall(_.isDigit)).map(_.toInt).take(3)
+               |      val xs = prefix ++ Seq.fill(3 - prefix.length)(0)
+               |      (xs(0), xs(1), xs(2))
+               |    }
+               |
+               |    val preferScala2: Command = Command.command("preferScala2") { state =>
+               |      val (structure, data) = {
+               |        val extracted = Project.extract(state)
+               |        (extracted.structure, extracted.structure.data)
+               |      }
+               |
+               |      val scala3Projects = structure.allProjectRefs.filter { project =>
+               |        (project / Keys.scalaVersion).get(data).exists(_.startsWith("3."))
+               |      }
+               |
+               |      val (scala2Versions, crossScala2And3ProjectsCount) = scala3Projects.foldLeft((Set.empty[String], 0)) {
+               |        case ((scala2Versions, crossCount), project) =>
+               |          val projectScala2Versions = (project / Keys.crossScalaVersions).get(data).getOrElse(Seq.empty).filter(_.startsWith("2."))
+               |          (scala2Versions ++ projectScala2Versions, crossCount + (if (projectScala2Versions.nonEmpty) 1 else 0))
+               |      }
+               |
+               |      // We can only do this when all sbt projects cross-compile to Scala 2 & Scala 3
+               |      // See https://youtrack.jetbrains.com/issue/SCL-22619/
+               |      val canSetScala2VersionGlobally = crossScala2And3ProjectsCount > 0 &&
+               |        scala3Projects.length == crossScala2And3ProjectsCount
+               |      if (canSetScala2VersionGlobally) {
+               |        "++" + scala2Versions.maxBy(numbersOf) :: state
+               |      } else {
+               |        state
+               |      }
+               |    }
+               |
+               |    xs.filterNot(_.nameOption.contains("preferScala2")) :+ preferScala2
+               |}""".stripMargin.raw
+      )
 
       final val sharedAggSettings = outOfSource ++ Seq(
         """SettingKey[Boolean]("ide-skip-project")""" := true,