Exclude scala-xml from transitive dependencies of partest.

As discovered here:
#6 (comment)
we would have two different versions of scala-xml on test classpath:

  1. scala-xml classes compiled from source (we are in scala-xml
     project)
  2. scala-xml transitive dependencies of partest

Duplicated classpath entries are always dangerous. I implemented a
set of exclude rules that get rid of transitive dependencies of
scala-xml. This fixes the problem of JUnit using wrong version of
scala-xml for testing (now there's only one choice).

Also, this allowed me to to enable conflictWarning again. They are
very useful to warn us against dangerous situations that almost always
result in broken classpath (as we just learned).

Author: gkossakowski

build.sbt

@@ -88,16 +88,34 @@ TestKeys.partestVersion := "1.0.0-RC6"
 // so that it can link to the compiler/lib we're using (testing)
 // NOTE: not sure why, but the order matters (maybe due to the binary version conflicts for xml/parser combinators pulled in for scaladoc?)
 libraryDependencies ++= (
-  if (TestKeys.includeTestDependencies.value)
-    Seq("org.scala-lang.modules" %% "scala-partest-interface" % "0.2"                         % "test",
-        "org.scala-lang.modules" %% "scala-partest"           % TestKeys.partestVersion.value % "test")
+  if (TestKeys.includeTestDependencies.value) {
+    /**
+     * Exclude all transitive dependencies of partest that include scala-.xml.
+     * This way we avoid having two (or more) versions of scala-xml on a classpath.
+     * This fixes problem described here:
+     * https://github.com/scala/scala-xml/pull/6#issuecomment-26614894
+     *
+     * Note that we are using ModuleID.exclude instead of more flexible ModuleID.excludeAll
+     * (describe here: http://www.scala-sbt.org/release/docs/Detailed-Topics/Library-Management#exclude-transitive-dependencies)
+     * because only plain excludes are incorporated in generated pom.xml. There are two ways
+     * to address this problem:
+     *
+     *   1. Figure out how to depend on partest in non-transitive way: not include that dependency
+     *      in generated pom.xml for scala-xml.
+     *   2. Declare dependencies in partest as provided so they are not includeded transitively.
+     *
+     */
+    def excludeScalaXml(dep: ModuleID): ModuleID =
+      dep.exclude("org.scala-lang.modules", "scala-xml_2.11.0-M5").
+      exclude("org.scala-lang.modules", "scala-xml_2.11.0-M4").
+      exclude("org.scalacheck", "scalacheck_2.11.0-M5")
+    Seq("org.scala-lang.modules" % "scala-partest-interface_2.11.0-M5" % "0.2"                         % "test",
+        "org.scala-lang.modules" % "scala-partest_2.11.0-M5"           % TestKeys.partestVersion.value % "test").
+      map(excludeScalaXml)
+  }
   else Seq.empty
 )
 
-
-// necessary for partest -- see comments in its build.sbt
-conflictWarning ~= { _.copy(failOnConflict = false) }
-
 fork in Test := true
 
 javaOptions in Test += "-Xmx1G"