According to http://www.scala-sbt.org/0.13.0/docs/Howto/generatefiles.html:

sourceGenerators in Compile += <your Task[Seq[File]] here>

But this simply doesn't work. It's obvious why when comparing with the 0.12.4 version, which gives:

sourceGenerators in Compile <+= <your Task[Seq[File]] here>

Now, that's just not the kind of change which happened between 0.12 and 0.13. Edit: I spoke a bit too soon; you can replace <+= with += if the type on the right changes, and loses the Initialize part, but it's not the case here, probably because it's a setting containing a task.

This is visible in the example. Example given for 0.12.4:

sourceGenerators in Compile <+= sourceManaged in Compile map { dir =>
  val file = dir / "demo" / "Test.scala"
  IO.write(file, """object Test extends App { println("Hi") }""")
  Seq(file)
}

Example given for 0.13.0, but non-working:

sourceGenerators in Compile += Def.task {
  val file = (sourceManaged in Compile).value / "demo" / "Test.scala"
  IO.write(file, """object Test extends App { println("Hi") }""")
  Seq(file)
}

After replacing += with <+=, we get a example that works:

sourceGenerators in Compile <+= Def.task {
  val file = (sourceManaged in Compile).value / "demo" / "Test.scala"
  IO.write(file, """object Test extends App { println("Hi") }""")
  Seq(file)
}

Error from the given 0.13.0 code (not very self-explanatory, I'd say):

> reload
[info] Loading project definition from /Users/pgiarrusso/Documents/Research/PhD/Teaching/SLE2013/public-material-repo/lectures/sbt-sample/project
/Users/pgiarrusso/Documents/Research/PhD/Teaching/SLE2013/public-material-repo/lectures/sbt-sample/build.sbt:8: error: No implicit for Append.Value[Seq[sbt.Task[Seq[java.io.File]]], sbt.std.FullInstance.M[Seq[java.io.File]]] found,
  so sbt.std.FullInstance.M[Seq[java.io.File]] cannot be appended to Seq[sbt.Task[Seq[java.io.File]]]
sourceGenerators in Compile += Def.task {
                            ^
[error] Type error in expression

I'd open a further bug saying "example code in documentation is not tested", but I don't know if you'd care and/or if the technology for fixing it exists.