diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index d2506a32a..d262342ca 100755
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -374,7 +374,7 @@ object Defaults extends BuildCommon {
   lazy val projectTasks: Seq[Setting[_]] = Seq(
     cleanFiles := cleanFilesTask.value,
     cleanKeepFiles := historyPath.value.toVector,
-    clean := IO.delete(cleanFiles.value),
+    clean := (Def.task { IO.delete(cleanFiles.value) } tag (Tags.Clean)).value,
     consoleProject := consoleProjectTask.value,
     watchTransitiveSources := watchTransitiveSourcesTask.value,
     watch := watchSetting.value
@@ -692,10 +692,15 @@ object Defaults extends BuildCommon {
     Def.task {
       Tests.discover(loadedTestFrameworks.value.values.toList, compile.value, streams.value.log)._1
     }
-  def defaultRestrictions: Initialize[Seq[Tags.Rule]] = parallelExecution { par =>
-    val max = EvaluateTask.SystemProcessors
-    Tags.limitAll(if (par) max else 1) :: Tags.limit(Tags.ForkedTestGroup, 1) :: Nil
-  }
+  def defaultRestrictions: Initialize[Seq[Tags.Rule]] =
+    Def.setting {
+      val par = parallelExecution.value
+      val max = EvaluateTask.SystemProcessors
+      Tags.limitAll(if (par) max else 1) ::
+      Tags.limit(Tags.ForkedTestGroup, 1) ::
+      Tags.exclusiveGroup(Tags.Clean) ::
+      Nil
+    }
 
   lazy val packageBase: Seq[Setting[_]] = Seq(
     artifact := Artifact(moduleName.value)
diff --git a/main/src/main/scala/sbt/Tags.scala b/main/src/main/scala/sbt/Tags.scala
index 4ab6b5fe0..28ce79f66 100644
--- a/main/src/main/scala/sbt/Tags.scala
+++ b/main/src/main/scala/sbt/Tags.scala
@@ -13,6 +13,7 @@ object Tags {
   val Test = Tag("test")
   val Update = Tag("update")
   val Publish = Tag("publish")
+  val Clean = Tag("clean")
 
   val CPU = Tag("cpu")
   val Network = Tag("network")
@@ -24,7 +25,7 @@ object Tags {
    * Describes a restriction on concurrently executing tasks.
    * A Rule is constructed using one of the Tags.limit* methods.
    */
-  sealed trait Rule { // TODO: make this an abstract class for 0.14
+  abstract class Rule {
     def apply(m: TagMap): Boolean
     def ||(r: Rule): Rule = new Or(this, r)
     def &&(r: Rule): Rule = new And(this, r)