warn when there is an sbt version conflict. fixes #80

ivy/ConflictWarning.scala

@@ -0,0 +1,20 @@
+package sbt
+
+	import DependencyFilter._
+
+final case class ConflictWarning(filter: ModuleFilter, group: ModuleID => String, level: Level.Value, failOnConflict: Boolean)
+object ConflictWarning
+{
+	def default: ConflictWarning = ConflictWarning( moduleFilter(organization = GlobFilter("org.scala-tools.sbt") | GlobFilter("org.scala-lang")), (_: ModuleID).organization, Level.Warn, false)
+
+	def apply(config: ConflictWarning, report: UpdateReport, log: Logger)
+	{
+		val conflicts = IvyActions.groupedConflicts(config.filter, config.group)(report)
+		if(!conflicts.isEmpty)
+			log.log(config.level, "Potentially incompatible versions specified:")
+		for( (label, versions) <- conflicts )
+			log.log(config.level, "   " + label + ": " + versions.mkString(", "))
+		if(config.failOnConflict && !conflicts.isEmpty)
+			error("Conflicts in " + conflicts.map(_._1).mkString )
+	}
+}

ivy/DependencyFilter.scala

@@ -32,6 +32,7 @@ object DependencyFilter extends DependencyFilterExtra
 	implicit def fnToModuleFilter(f: ModuleID => Boolean): ModuleFilter = new ModuleFilter { def apply(m: ModuleID) = f(m) }
 	implicit def fnToArtifactFilter(f: Artifact => Boolean): ArtifactFilter = new ArtifactFilter { def apply(m: Artifact) = f(m) }
 	implicit def fnToConfigurationFilter(f: String => Boolean): ConfigurationFilter = new ConfigurationFilter { def apply(c: String) = f(c) }
+	implicit def subDepFilterToFn[Arg](f: SubDepFilter[Arg, _]): Arg => Boolean = f apply _
 }
 trait DependencyFilter
 {

ivy/IvyActions.scala

@@ -131,6 +131,18 @@ object IvyActions
 			}
 		}
 
+	def groupedConflicts[T](moduleFilter: ModuleFilter, grouping: ModuleID => T)(report: UpdateReport): Map[T, Set[String]] =
+		report.configurations.flatMap { confReport =>
+			val evicted = confReport.evicted.filter(moduleFilter)
+			val evictedSet = evicted.map( m => (m.organization, m.name) ).toSet
+			val conflicted = confReport.allModules.filter( mod => evictedSet( (mod.organization, mod.name) ) )
+			grouped(grouping)(conflicted ++ evicted)
+		} toMap;
+
+	def grouped[T](grouping: ModuleID => T)(mods: Seq[ModuleID]): Map[T, Set[String]] =
+		mods groupBy(grouping) mapValues(_.map(_.revision).toSet)
+
+
 	def transitiveScratch(ivySbt: IvySbt, label: String, config: GetClassifiersConfiguration, log: Logger): UpdateReport =
 	{
 			import config.{configuration => c, id, ivyScala, modules => deps}