title: Reference to undefined setting error with custom task in SBT?
body: <p>I'm creating a task in SBT that will create a properly-formatted classpath, but I get an undefined setting error when I try to <code>compile</code>. The error is not useful enough to tell me what setting is undefined:</p>
  
  <pre><code>[error] Reference to undefined setting: 
  [error] 
  [error]   *:fullClasspath from *:fcp
  [error]  
  </code></pre>
  
  <p>This is for the following build configuration:</p>
  
  <pre><code>import sbt._
  import Keys._
  
  object TestBuild extends Build {
      override lazy val settings = super.settings ++ Seq(
          name := "blah", scalaVersion := "2.10.2"
      )
  
      lazy val fcp = TaskKey[String]("fcp", "create formatted classpath")
      lazy val fcpTask = fcp &lt;&lt;= fullClasspath map { tasks =&gt;
          val cp = tasks.map(_.data.getName + ":")
          s"[Classpath]$cp[/Classpath]"
      }
  
      lazy val project = Project(
          "project", file("."), settings = settings ++ Seq(fcpTask)
      )
  }
  </code></pre>