sbt says "not a valid command" but plugin enabled - sbt

Question

I have a plugin called sonar, which is developed and published as an AutoPlugin:
object Sonar extends AutoPlugin {
object autoImport {
lazy val sonar = taskKey[Unit]("sonar")
}
import autoImport._
override def trigger = allRequirements
lazy val sonarTask = Def.task {
<snip task code here which runs sonarqube scanner>
}
}
We then have a project which uses that plugin, with plugins.sbt that looks like this:
resolvers ++= Seq(
"Nexus Snapshot repo" at "url here"
)
credentials += Credentials(Path.userHome / ".ivy2" / ".credentials")
addSbtPlugin("packagename" % "sonar" % "1.0-SNAPSHOT")
And build.sbt like this:
name := "hashing-library"
organization := "org name here"
scalaVersion := "2.12.6"
autoScalaLibrary := false
crossPaths := false
resolvers += "Nexus Release repo" at "https://nexusurl/content/repositories/releases/"
resolvers += "nexus Snapshot repo" at "https://nexusurl/content/repositories/snapshots/"
credentials += Credentials(Path.userHome / ".ivy2" / ".credentials")
// Publishing options:
publishMavenStyle := true
publishArtifact in Test := false
pomIncludeRepository := { x => false }
publishTo := {
val nexus = "nexus url here"
if (isSnapshot.value)
Some("sonatype-snapshots" at nexus + "content/repositories/snapshots")
else
Some("sonatype-releases" at nexus + "content/repositories/releases")
}
When I try to run sbt plugins, it says the plugin is enabled:
<snip output>
packagename.sonar.Sonar: enabled in hashingLibrary
So why is it that my plugin is enabled, but I cannot run sbt sonar? When I do, it says:
[info] Set current project to hashing-library (in build file:/home/work/hashing-library/)
[error] Not a valid command: sonar
[error] Not a valid project ID: sonar
[error] Expected ':'
[error] Not a valid key: sonar
[error] sonar
[error]
(Obviously org names and url's have been removed to protect confidentiality of my client, but hopefully that doesn't impact my question!)

Answer 1

The missing part of your plugin is the connection between the sonar task key and the sonarTask task implementation, i.e. you need to say somewhere that the key is set to the value of the implementation. You normally do it by overriding project settings in the plugin:
override def projectSettings = Seq(
sonar := sonarTask.value
)