Adds conflict check at the end to enforce exclusion requirements.

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

@@ -160,12 +160,19 @@ object Plugins extends PluginsFunctions
 				log.debug(s"deducing auto plugins based on known facts ${knowlege0.toString} and clauses ${clauses.toString}")
 				Logic.reduce(clauses, (flattenConvert(requestedPlugins) ++ convertAll(alwaysEnabled)).toSet) match {
 					case Left(problem) => throw AutoPluginException(problem)
-					case Right(results0) =>
-						log.debug(s"  :: deduced result: ${results0}")
-						val plugins = results0.ordered map { a =>
+					case Right(results) =>
+						log.debug(s"  :: deduced result: ${results}")
+						val selectedAtoms: List[Atom] = results.ordered
+						val selectedPlugins = selectedAtoms map { a =>
 							byAtomMap.getOrElse(a, throw AutoPluginException(s"${a} was not found in atom map."))
 						}
-						val retval = topologicalSort(plugins, log)
+						val forbidden: Set[AutoPlugin] = (selectedPlugins flatMap { Plugins.asExclusions }).toSet
+						val c = selectedPlugins.toSet & forbidden
+						if (!c.isEmpty) {
+							val listString = (c map {_.label}).mkString(", ")
+							throw AutoPluginException(s"Contradiction in selected plugins.  These plguins were both included and excluded: ${listString}")
+						}
+						val retval = topologicalSort(selectedPlugins, log)
 						log.debug(s"  :: sorted deduced result: ${retval.toString}")
 						retval
 				}
@@ -245,6 +252,9 @@ object Plugins extends PluginsFunctions
 	private[sbt] def asRequirements(ap: AutoPlugin): List[AutoPlugin] = flatten(ap.requires).toList collect {
 		case x: AutoPlugin => x
 	}
+	private[sbt] def asExclusions(ap: AutoPlugin): List[AutoPlugin] = flatten(ap.requires).toList collect {
+		case Exclude(x) => x
+	}
 	private[this] def flattenConvert(n: Plugins): Seq[Literal] = n match {
 		case And(ns) => convertAll(ns)
 		case b: Basic => convertBasic(b) :: Nil

sbt/src/sbt-test/project/auto-plugins-conflict/build.sbt

@@ -0,0 +1,4 @@
+// with S selected, Q is loaded automatically, which in turn selects R
+lazy val projA = project.addPlugins(S)
+
+check := ()

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

@@ -0,0 +1,35 @@
+	import sbt._
+
+object AI extends AutoImport
+{
+	trait EmptyAutoPlugin extends AutoPlugin {
+		def requires = empty
+		def trigger = noTrigger
+	}
+	object A extends EmptyAutoPlugin
+	object B extends EmptyAutoPlugin
+
+	lazy val check = settingKey[Unit]("Verifies settings are as they should be.")
+}
+
+	import AI._
+
+object Q extends AutoPlugin
+{
+	def requires: Plugins = A && B
+	def trigger = allRequirements
+}
+
+object R extends AutoPlugin
+{
+	def requires = Q
+	def trigger = allRequirements
+}
+
+// This is an opt-in plugin with a requirement
+// Unless explicitly loaded by the build user, this will not be activated.
+object S extends AutoPlugin
+{
+	def requires = Q && !R
+	def trigger = noTrigger
+}

sbt/src/sbt-test/project/auto-plugins-conflict/test

@@ -0,0 +1 @@
+-> check