ServiceWithDependency.php

<?php

namespace Drupal\new_dependency_test;


/**
 * Service that gets the other service of the same module injected.
 *
 * This service indirectly depends on a not-yet-defined service.
 */
class ServiceWithDependency {
    
    /**
     * The injected service.
     *
     * @var \Drupal\new_dependency_test\InjectedService
     */
    protected $service;
    
    /**
     * ServiceWithDependency constructor.
     *
     * @param \Drupal\new_dependency_test\InjectedService|null $service
     *   The service of the same module which has the new dependency.
     */
    public function __construct(?InjectedService $service = NULL) {
        $this->service = $service;
    }
    
    /**
     * Gets a greeting from the injected service and adds to it.
     *
     * @return string
     *   The greeting.
     */
    public function greet() {
        if (isset($this->service)) {
            return $this->service
                ->greet() . ' World';
        }
        return 'Sorry, no service.';
    }

}