tasks.register<ExampleTask>("example")

abstract class ExampleTask : DefaultTask() {
    @TaskAction
    fun initialize() {
        println("Hello World")
    }
}

./gradlew example

Could not create task :example
    Could not create task of type ExampleTask
        Class Build_gradle.ExampleTask is a non-static inner class.

class ExampleTask

val x = file(".")
abstract class ExampleTask : DefaultTask() {
    init {
        x // bad
        file(".") // also bad

Task.project

project.rootProject

rootProject

project

so

project.rootProject

is OK,

using

project

at execution time is not allowed with configuration cache,

what are you trying to do?

tasks.register("example", ExampleTask::class) {
    outputFile.set(rootProject.file("output.txt"))
}

abstract class ExampleTask : DefaultTask() {
    @get:OutputFile
    abstract val outputFile: RegularFileProperty

    @TaskAction
    fun initialize() {
        outputFile.get().getAsFile().writeText("Hello World")
    }
}

And I choose not to put it in buildSrc since it re-invalidates all modules for every single change

buildSrc