How to upcast webform submission in custom routes resp. controller?

Question

Using this route:

confirm:
  path: '/node/{node}/submission/{webform_submission}/confirm'
  defaults:
    _controller: '\Drupal\mymodule\Controller\MymoduleController::confirm'
    _title: 'Confirm your submission'
  requirements:
    _permission: 'confirm webform submission'

And visiting node/15/submission/15/confirm I get the error:

RuntimeException: Controller "Drupal\mymodule\Controller\MymoduleController::confirm()" requires that you provide a value for the "$submission" argument. Either the argument is nullable and no null value has been provided, no default value has been provided or because there is a non optional argument after this one. in Symfony\Component\HttpKernel\Controller\ArgumentResolver->getArguments()

What am I doing wrong? Why doesnt it load the submission 15? It does exist.

Edit: this is my Controller, the second argument is always null:

<?php

namespace Drupal\mymodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Entity\Controller\EntityViewController;
use Symfony\Component\HttpFoundation\Response;
use Drupal\Core\Entity\EntityInterface;
use Drupal\webform\WebformSubmissionInterface;
use Drupal\webform\WebformSubmission;

/**
 * @todo
 */
// class MymoduleController extends ControllerBase { // doesnt make a difference
class MymoduleController extends EntityViewController {
  public function confirm(EntityInterface $node = null, EntityInterface $submission = null) {

    debug($submission);

    $response = new Response( 'test' );

    return $response;
  }
}

Answer 1

You probably just need to declare the parameter type:

confirm:
  path: '/node/{node}/submission/{webform_submission}/confirm'
  defaults:
    _controller: '\Drupal\mymodule\Controller\MymoduleController::confirm'
    _title: 'Confirm your submission'
  requirements:
    _permission: 'confirm webform submission'
  options:
    parameters:
      webform_submission:
        type: entity:webform_submission

The parameter names also need to match, so update your controller accordingly:

public function confirm(EntityInterface $node = null, EntityInterface $webform_submission = null) {