Override current project's scalaVersion on ^^

Both the default settings and ^^ together sets the correct scalaVersion based on `sbtVersion in pluginCrossBuild`, but frequently people set up `scalaVersion` on sbt plugin's subproject, which disables the feature.
This change appends the scalaVersionSetting on ^^ so scalaVersion gets switched to 2.12.2 on ^^ 1.0.0-RC2 etc.

Fixes #3205

main/src/main/scala/sbt/Defaults.scala

@@ -238,17 +238,13 @@ object Defaults extends BuildCommon {
     }
   )
 
+  // This is included into JvmPlugin.projectSettings
   def compileBase = inTask(console)(compilersSetting :: Nil) ++ compileBaseGlobal ++ Seq(
     incOptions := incOptions.value.withNewClassfileManager(
       sbt.inc.ClassfileManager.transactional(crossTarget.value / "classes.bak", sbt.Logger.Null)),
     scalaInstance := scalaInstanceTask.value,
     crossVersion := (if (crossPaths.value) CrossVersion.binary else CrossVersion.Disabled),
-    scalaVersion := {
+    scalaVersion := PluginCross.scalaVersionSetting.value,
-      val scalaV = scalaVersion.value
-      val sv = (sbtBinaryVersion in pluginCrossBuild).value
-      if (sbtPlugin.value) scalaVersionFromSbtBinaryVersion(sv)
-      else scalaV
-    },
     sbtBinaryVersion in pluginCrossBuild := binarySbtVersion((sbtVersion in pluginCrossBuild).value),
     crossSbtVersions := Vector((sbtVersion in pluginCrossBuild).value),
     crossTarget := makeCrossTarget(target.value,
@@ -284,14 +280,6 @@ object Defaults extends BuildCommon {
     derive(scalaBinaryVersion := binaryScalaVersion(scalaVersion.value))
   ))
 
-  private[sbt] def scalaVersionFromSbtBinaryVersion(sv: String): String =
-    VersionNumber(sv) match {
-      case VersionNumber(Seq(0, 12, _*), _, _) => "2.9.2"
-      case VersionNumber(Seq(0, 13, _*), _, _) => "2.10.6"
-      case VersionNumber(Seq(1, 0, _*), _, _)  => "2.12.2"
-      case _                                   => sys.error(s"Unsupported sbt binary version: $sv")
-    }
-
   def makeCrossSources(scalaSrcDir: File, javaSrcDir: File, sv: String, cross: Boolean): Seq[File] = {
     if (cross)
       Seq(scalaSrcDir.getParentFile / s"${scalaSrcDir.name}-$sv", scalaSrcDir, javaSrcDir)

main/src/main/scala/sbt/PluginCross.scala

@@ -15,6 +15,7 @@ import Scope.GlobalScope
 import Def.ScopedKey
 import CommandStrings._
 import Cross.{ spacedFirst, requireSession }
+import Configurations._
 
 /**
  * Module responsible for plugin cross building.
@@ -39,7 +40,10 @@ private[sbt] object PluginCross {
         val x = Project.extract(state)
         import x._
         state.log.info(s"Setting `sbtVersion in pluginCrossBuild` to $version")
-        val add = (sbtVersion in GlobalScope in pluginCrossBuild :== version) :: Nil
+        val add = List(sbtVersion in GlobalScope in pluginCrossBuild :== version) ++
+          inConfig(Compile)(List(scalaVersion := scalaVersionSetting.value)) ++
+          inConfig(Test)(List(scalaVersion := scalaVersionSetting.value))
+
         val cleared = session.mergeSettings.filterNot(crossExclude)
         val newStructure = Load.reapply(cleared ++ add, structure)
         Project.setProject(session, newStructure, command :: state)
@@ -69,4 +73,19 @@ private[sbt] object PluginCross {
         else versions.map(PluginSwitchCommand + " " + _ + " " + command) ::: current ::: state
     }
   }
+
+  def scalaVersionSetting: Def.Initialize[String] = Def.setting {
+    val scalaV = scalaVersion.value
+    val sv = (sbtBinaryVersion in pluginCrossBuild).value
+    if (sbtPlugin.value) scalaVersionFromSbtBinaryVersion(sv)
+    else scalaV
+  }
+
+  def scalaVersionFromSbtBinaryVersion(sv: String): String =
+    VersionNumber(sv) match {
+      case VersionNumber(Seq(0, 12, _*), _, _) => "2.9.2"
+      case VersionNumber(Seq(0, 13, _*), _, _) => "2.10.6"
+      case VersionNumber(Seq(1, 0, _*), _, _)  => "2.12.2"
+      case _                                   => sys.error(s"Unsupported sbt binary version: $sv")
+    }
 }