How can I use sbt commands like clean and compile in my custom sbtplugin

I have the following, minimal, custom sbt plugin, where I'd like to implement (override) the standard clean command to do the same action as myTask.

package nigeleke.sbt

import sbt.*
import Keys.*

import scala.sys.process.*

object MyPlugin extends AutoPlugin {

  object autoImport {
    val myTask = taskKey[Unit]("Do something.")
  }

  import autoImport._

  override def requires = empty

  override def trigger = noTrigger

  override lazy val projectSettings = Seq(
    myTask := {
      println(s"project: ${thisProject.value.id}  plugins: ${thisProject.value.plugins}")
    },
    clean  := clean.dependsOn(myTask).value
  )

}

When I enable this plugin a client project, the myTask command works as expected. I would also like clean to invoke the same task, but no output is forthcoming:

> sbt
[info] welcome to sbt 1.8.2 (Eclipse Adoptium Java 17.0.6)
[info] loading global plugins from ... \.sbt\1.0\plugins
[info] loading settings for project sbt-example-client-build from plugins.sbt ...
[info] loading project definition from ... \sbt-example-client\project
[info] loading settings for project root from build.sbt,version.sbt ...
[info] set current project to sbt-example-client (in build file:/ ... /sbt-example-client/)
[info] sbt server started at local:sbt-server-c377c6b81cca5b432adb
[info] started sbt server
sbt:sbt-example-client> myTask
project: client  plugins: nigeleke.sbt.MyPlugin
[success] Total time: 0 s, completed 13 Apr 2023, 5:05:18 pm
sbt:sbt-example-client> clean
[success] Total time: 0 s, completed 13 Apr 2023, 5:05:21 pm
sbt:sbt-example-client>

I'm failing to find the correct syntax to invoke it, so any pointers are appreciated.

Solution

The thing is that clean is defined in a default built-in sbt plugin JvmPlugin

https://github.com/sbt/sbt/blob/v1.8.2/main/src/main/scala/sbt/plugins/JvmPlugin.scala#L44-L46

object JvmPlugin extends AutoPlugin {
  ...
  override lazy val projectSettings: Seq[Setting[_]] =
      ... ++
      Defaults.baseTasks ++    // (1)
      Defaults.compileBase ++  // (2)
      Defaults.defaultConfigs  // (3)
}

https://github.com/sbt/sbt/blob/v1.8.2/main/src/main/scala/sbt/Defaults.scala

object Defaults extends BuildCommon { 

  lazy val baseTasks: Seq[Setting[_]] = projectTasks ++ ...

  lazy val projectTasks: Seq[Setting[_]] = Seq(
    ...
    clean :=                                 // (1)
      Def.taskDyn(Clean.task(resolvedScoped.value.scope, full = true)).value,
    ...
  )

  def compileBase = ... ++ Seq(
    ...
    clean := clean.dependsOn(cleanIvy).value, // (2)
    ...
  )

  lazy val defaultConfigs: Seq[Setting[_]] =
    inConfig(Compile)(compileSettings) ++ 
    inConfig(Test)(testSettings) ++ ...

  lazy val compileSettings: Seq[Setting[_]] = configSettings + ...

  lazy val testSettings: Seq[Setting[_]] = configSettings ++ ...

  lazy val configSettings: Seq[Setting[_]] = ... ++ configTasks ++ ...

  lazy val configTasks: Seq[Setting[_]] = ... ++ Seq(
    ...
    clean := (compileOutputs / clean).value,  // (3)
    ...
  )
}

You can't disable JvmPlugin because in such case sbt can't build a project.

So in order to override clean you should make your plugin depend on JvmPlugin (otherwise JvmPlugin definition of clean overrides yours)

object MyPlugin extends AutoPlugin {
  ...

  override def requires = JvmPlugin

Then any of the following options works:

clean := clean.dependsOn(myTask).value

clean := {
  myTask.value
  clean.value
}

clean := Def.sequential(myTask, clean).value

How can a default task be overridden from an AutoPlugin?

https://www.scala-sbt.org/1.x/docs/Custom-Settings.html#Implementing+a+task

SBT task dependsOn

SBT dependsOn usage - migration from 0.12 to 0.13

Per-project tasks in SBT

Sbt run command for multiple projects

Sequencing and overriding tasks in SBT

How to exclude files in a custom clean task?

How to execute clean task in dependent projects from a task?