No implicit for Append.Value[...] found with Def.task to generate files in SBT 0.13?

Question

I'm having an issue migrating my project to SBT 0.13.

For some reason, the snippet from Generate sources from the SBT documentation doesn't work for me.

Neither a simple .sbt build definition nor a Scala build definition work, unfortunately. Project definition is taken from the documentation:

name := "sbt-test"

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)
}

The compiler complains about the type error when compiling the project definition:

~/P/sbt-test ▶ sbt
[info] Loading global plugins from /Users/phearnot/.sbt/0.13/plugins
[info] Loading project definition from /Users/phearnot/Projects/sbt-test/project
/Users/phearnot/Projects/sbt-test/build.sbt:3: 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
Project loading failed: (r)etry, (q)uit, (l)ast, or (i)gnore? 

UPDATE: Now that AlexIv has pointed out the problem in my SBT file definition, I wonder what is the proper way to move it into a Scala build definition?

Answer 1

Change Def.task in your build.sbt (from gist) to Def.task[Seq[File]] or just leave task[Seq[File]] cause Def is automatically imported into build.sbt:

name := "sbt-test"

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

Then call compile in sbt. Test.scala will be generated in ./target/scala-2.10/src_managed/main/demo/Test.scala