}
            if (mutations.hasFileInputs) {
                throw new IllegalStateException("Task " + taskNode + " has both inputs and destroyables defined.  A task can define either inputs or destroyables, but not both.");
            }
            if (mutations.hasLocalState) {

                outputs.file('foo')
                destroyables.register('bar')
            }
        """
        file('foo') << 'foo'
        file('bar') << 'bar'

        expect:
        2.times {
            fails 'a'
            failure.assertHasDescription('Task :a has both outputs and destroyables defined.  A task can define either outputs or destroyables, but not both.')
        }
    }

        given:
        withParallelThreads(2)
        buildFile << """
            def dir = rootProject.file("dir")

            aPing.destroyables.register dir

            bPing.outputs.file dir
        """

        expect:
        2.times {
            blockingServer.expectConcurrent(1, ":aPing", ":bPing")
            run ":aPing", ":bPing"

    }

    private
    suspend fun WriteContext.writeDestroyablesOf(task: TaskInternal) {
        val destroyables = (task.destroyables as TaskDestroyablesInternal).registeredFiles
        if (destroyables.isEmpty) {
            writeBoolean(false)
        } else {
            writeBoolean(true)
            write(destroyables)
        }
    }

    private
    suspend fun ReadContext.readDestroyablesOf(task: TaskInternal) {

        private final List<Object> destroyables = new ArrayList<>();

        public GetDestroyablesVisitor(String beanName, FileCollectionFactory fileCollectionFactory) {
            this.beanName = beanName;
            this.fileCollectionFactory = fileCollectionFactory;
        }

        @Override
        public void visitDestroyableProperty(Object value) {
            destroyables.add(value);
        }

        output.contains "Input property 'beans.\$1'"
        output.contains "Input property 'beans.\$1.secondInput'"
    }

    @ToBeFixedForConfigurationCache(because = "task references another task")
    def "nested destroyables are discovered"() {
        buildFile << classesForNestedProperties()
        buildFile << """
            class MyDestroyer extends DefaultTask {
                @TaskAction void doStuff() {}
                @Nested

        _ * task.identityPath >> Path.path(services.identifier.buildPath).child("task")
        _ * task.taskIdentity >> TestTaskIdentities.create("task", DefaultTask, project)
        _ * task.destroyables >> Stub(TaskDestroyablesInternal)
        _ * task.localState >> Stub(TaskLocalStateInternal)
        _ * project.gradle >> services.gradle
        _ * project.owner >> projectState

        _ * mock.compareTo(_) >> { Task t -> name.compareTo(t.name) }
        _ * mock.outputs >> Stub(TaskOutputsInternal)
        _ * mock.inputs >> Stub(TaskInputsInternal)
        _ * mock.destroyables >> Stub(TaskDestroyablesInternal)
        _ * mock.localState >> Stub(TaskLocalStateInternal)
        _ * mock.path >> ":${name}"

        taskProperties.outputFileProperties.empty
        taskProperties.destroyableFiles.empty
    }

    def "registers destroyables"() {
        def destroyable = new File(temporaryFolder.file('destroyable').createFile().absolutePath)
        given:
        def task = expectTaskCreated(TaskWithDestroyable, destroyable)

        when:
        def taskProperties = DefaultTaskProperties.resolve(propertyWalker, fileCollectionFactory, task)

     *
     * @return The outputs. Never returns null.
     */
    @Internal
    TaskOutputs getOutputs();

    /**
     * <p>Returns the destroyables of this task.</p>
     * @return The destroyables.  Never returns null.
     *
     * @since 4.0
     */
    @Internal
    TaskDestroyables getDestroyables();

    /**
     * Returns the local state of this task.
     *