Append dummy argument to `-classpath` when it is actually empty.

Scala compiler's way of handling empty classpath argument is problematic.
This workaround appends a dummy classpath argument when the classpath is
actually empty. Fixes #269 (also see #82, #85).

compile/CompilerArguments.scala

@@ -18,7 +18,10 @@ final class CompilerArguments(scalaInstance: ScalaInstance, cp: ClasspathOptions
 	{
 		checkScalaHomeUnset()
 		val cpWithCompiler = finishClasspath(classpath)
-		val classpathOption = Seq("-classpath", if(cpWithCompiler.isEmpty) "" else absString(cpWithCompiler) )
+		// Scala compiler's treatment of empty classpath is troublesome (as of 2.9.1).
+		// We append a random dummy element as workaround.
+		val dummy = "dummy_" + Integer.toHexString(util.Random.nextInt)
+		val classpathOption = Seq("-classpath", if(cpWithCompiler.isEmpty) dummy else absString(cpWithCompiler))
 		val outputOption = Seq("-d", outputDirectory.getAbsolutePath)
 		options ++ outputOption ++ bootClasspathOption ++ classpathOption ++ abs(sources)
 	}
@@ -55,4 +58,4 @@ object CompilerArguments
 	def abs(files: Set[File]): Seq[String] = abs(files.toSeq)
 	def absString(files: Seq[File]): String = abs(files).mkString(File.pathSeparator)
 	def absString(files: Set[File]): String = absString(files.toSeq)
-}
+}

sbt/src/sbt-test/actions/doc/package.scala

@@ -0,0 +1,5 @@
+package pkg
+
+package object foo {
+  final val foo = "Foo"
+}

sbt/src/sbt-test/actions/doc/test

@@ -13,7 +13,9 @@ $ exists "target/api/java"
 
 > 'set sources in (Compile, doc) <<= sources in (Compile, doc) map { _.filterNot(_.getName endsWith ".java") }'
 
-> ; clean ; doc
+# compile task is superfluous. Since doc task preceded by compile task has been problematic due to scala
+# compiler's way of handling empty classpath. We have it here to test that our workaround works.
+> ; clean ; compile ; doc
 
 # pure scala project, only scaladoc at top level
 $ exists "target/api/index.js"