AutoPlugin should require JvmPlugin by default

Fixes #2082

main/src/main/scala/sbt/Plugins.scala

@@ -71,7 +71,7 @@ abstract class AutoPlugin extends Plugins.Basic with PluginsFunctions {
 
   /** This AutoPlugin requires the plugins the [[Plugins]] matcher returned by this method. See [[trigger]].
    */
-  def requires: Plugins = empty
+  def requires: Plugins = plugins.JvmPlugin
 
   val label: String = getClass.getName.stripSuffix("$")
 

main/src/main/scala/sbt/plugins/CorePlugin.scala

@@ -11,6 +11,7 @@ import Def.Setting
 object CorePlugin extends AutoPlugin {
   // This is included by default
   override def trigger = allRequirements
+  override def requires = empty
 
   override lazy val projectSettings: Seq[Setting[_]] =
     Defaults.coreDefaultSettings

main/src/test/scala/PluginCommandTest.scala

@@ -7,11 +7,11 @@ import org.specs2.mutable.Specification
 import sbt.internal._
 import sbt.internal.util.{ AttributeEntry, AttributeMap, ConsoleOut, GlobalLogging, MainLogging, Settings }
 
-object PluginCommandTestPlugin0 extends AutoPlugin
+object PluginCommandTestPlugin0 extends AutoPlugin { override def requires = empty }
 
 package subpackage {
 
-  object PluginCommandTestPlugin1 extends AutoPlugin
+  object PluginCommandTestPlugin1 extends AutoPlugin { override def requires = empty }
 
 }
 

main/src/test/scala/PluginsTest.scala

@@ -54,8 +54,8 @@ object AI
 	lazy val deducePlugin = Plugins.deducer(allPlugins)
 	lazy val log = Logger.Null
 
-	object A extends AutoPlugin
-	object B extends AutoPlugin
+	object A extends AutoPlugin { override def requires = empty }
+	object B extends AutoPlugin { override def requires = empty }
 
 	object Q extends AutoPlugin
 	{

sbt/src/sbt-test/project/auto-plugins-default-requires-jvmplugin/build.sbt

@@ -0,0 +1,14 @@
+val test123 = project in file(".") enablePlugins TestP settings(
+  resourceGenerators in Compile <+= Def.task {
+    streams.value.log info "resource generated in settings"
+    Nil
+  }
+)
+
+TaskKey[Unit]("check") := {
+  val last = IO read (BuiltinCommands lastLogFile state.value).get
+  def assertContains(expectedString: String) =
+    if (!(last contains expectedString)) sys error s"Expected string $expectedString to be present"
+  assertContains("resource generated in settings")
+  assertContains("resource generated in plugin")
+}

sbt/src/sbt-test/project/auto-plugins-default-requires-jvmplugin/project/TestP.scala

@@ -0,0 +1,10 @@
+import sbt._, syntax._, Keys._
+
+object TestP extends AutoPlugin {
+  override def projectSettings: Seq[Setting[_]] = Seq(
+    resourceGenerators in Compile <+= Def.task {
+      streams.value.log info "resource generated in plugin"
+      Nil
+    }
+  )
+}

sbt/src/sbt-test/project/auto-plugins-default-requires-jvmplugin/test

@@ -0,0 +1,4 @@
+# Test case from https://github.com/sbt/sbt/issues/2081, adapted
+
+> test
+> check

sbt/src/sbt-test/project/auto-plugins/project/Q.scala

@@ -3,9 +3,9 @@ package sbttest // you need package http://stackoverflow.com/questions/9822008/
 	import sbt._, syntax._, Keys._
 	import java.util.concurrent.atomic.{AtomicInteger => AInt}
 
-	object A extends AutoPlugin
-	object B extends AutoPlugin
-	object E extends AutoPlugin
+	object A extends AutoPlugin { override def requires: Plugins = empty }
+	object B extends AutoPlugin { override def requires: Plugins = empty }
+	object E extends AutoPlugin { override def requires: Plugins = empty }
 
 object Imports
 {
@@ -20,6 +20,7 @@ object Imports
 
 object OrgPlugin extends AutoPlugin {
 	override def trigger = allRequirements
+	override def requires: Plugins = empty
 	override def projectSettings = Seq(
       organization := "override"
 	)