Avoid creating ScalaInstance when not used

sbt-dotty/sbt-test/sbt-dotty/scala2/build.sbt

@@ -0,0 +1 @@
+ThisBuild / scalaVersion := "2.12.8"

sbt-dotty/sbt-test/sbt-dotty/scala2/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/sbt-dotty/scala2/src/main/scala/hello/Hello.scala

@@ -0,0 +1,7 @@
+package hello
+/** Hello, world! */
+object Hello {
+  def main(args: Array[String]): Unit = {
+    println("Hello Scala 2")
+  }
+}

sbt-dotty/sbt-test/sbt-dotty/scala2/test

@@ -0,0 +1,2 @@
+> run
+> doc

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -142,6 +142,8 @@ object DottyPlugin extends AutoPlugin {
     }
   )
 
+  // https://github.com/sbt/sbt/issues/3110
+  val Def = sbt.Def
   override def projectSettings: Seq[Setting[_]] = {
     Seq(
       isDotty := scalaVersion.value.startsWith("0."),
@@ -293,13 +295,10 @@ object DottyPlugin extends AutoPlugin {
             allJars
           )
         }
-        else Def.task {
-          // This should really be `old` with `val old = scalaInstance.value`
-          // above, except that this would force the original definition of the
-          // `scalaInstance` task to be computed when `isDotty` is true, which
-          // would fail because `managedScalaInstance` is false.
-          Defaults.scalaInstanceTask.value
-        }
+        else
+          // This dereferences the Initialize graph, but keeps the Task unevaluated,
+          // so its effect gets fired only when isDotty.value evaluates to false. yay monad.
+          Def.valueStrict { scalaInstance.taskValue }
       }.value,
 
       // Because managedScalaInstance is false, sbt won't add the standard library to our dependencies for us