MatchError when using build transformers

[rtimush]

Build.scala:

  override def buildLoaders = BuildLoader.transform(_.unit) :: Nil

Error:

scala.MatchError: Stream(sbt.BuildLoader$Components@2ed59988, ?) (of class scala.collection.immutable.Stream$Cons)
at sbt.Load$.addResolvers(Load.scala:273)
at sbt.Load$.loadAll(Load.scala:314)
at sbt.Load$.loadURI(Load.scala:264)
at sbt.Load$.load(Load.scala:260)
at sbt.Load$.load(Load.scala:251)
at sbt.Load$.apply(Load.scala:134)
at sbt.Load$.defaultLoad(Load.scala:37)
at sbt.BuiltinCommands$.doLoadProject(Main.scala:473)
at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:467)
at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:467)
at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:60)
at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:60)
at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:62)
at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:62)
at sbt.Command$.process(Command.scala:95)
at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:100)
at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:100)
at sbt.State$$anon$1.process(State.scala:179)
at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:100)
at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:100)
at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:18)
at sbt.MainLoop$.next(MainLoop.scala:100)
at sbt.MainLoop$.run(MainLoop.scala:93)
at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:71)
at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:66)
at sbt.Using.apply(Using.scala:25)
at sbt.MainLoop$.runWithNewLog(MainLoop.scala:66)
at sbt.MainLoop$.runAndClearLast(MainLoop.scala:49)
at sbt.MainLoop$.runLoggedLoop(MainLoop.scala:33)
at sbt.MainLoop$.runLogged(MainLoop.scala:25)
at sbt.StandardMain$.runManaged(Main.scala:57)
at sbt.xMain.run(Main.scala:29)
at xsbt.boot.Launch$$anonfun$run$1.apply(Launch.scala:109)
at xsbt.boot.Launch$.withContextLoader(Launch.scala:129)
at xsbt.boot.Launch$.run(Launch.scala:109)
at xsbt.boot.Launch$$anonfun$apply$1.apply(Launch.scala:36)
at xsbt.boot.Launch$.launch(Launch.scala:117)
at xsbt.boot.Launch$.apply(Launch.scala:19)
at xsbt.boot.Boot$.runImpl(Boot.scala:44)
at xsbt.boot.Boot$.main(Boot.scala:20)
at xsbt.boot.Boot.main(Boot.scala)

[jsuereth]

Appears to be caused by a swap form List[_] to Stream[_] NOT failing the pattern matcher. We should probably start compiling with warnings enabled just to be able to catch some of these...

[rtimush]

As far as I understood the problem is the type is Seq but it is matched as if it is List, although there are other Seq implementations.
Does scala compiler has a warning for this? REPL doesn't show me a warning in this case.

[jsuereth]

hmm... I had thought it did. A lot of sbt code is written with List expected for all collections so that :: pattern matching works. While we could swap to the +:, I was hoping we could find a way to get a compiler warning on these instances....