Make theme_status_report compatible with drupal_render()

+ $status_report = drupal_render($status); $status_report .= t('Check the messages and <a href="!url">try again</a>.', array('!url' => check_url(drupal_requirements_url($severity))));

No new line here after the first ;

+ return drupal_render($status);

Extra spaces after return.

+ //$helpful_links = update_helpful_links();

Need a space between // and $

-  $output .= theme('links', array('links' => update_helpful_links()));
+  //$helpful_links = update_helpful_links();
+  $links = array(
+    '#theme' => 'links',
+    '#links' => update_helpful_links(),
+  );
+  $output .= drupal_render($links);

This has nothing to do with theme_status_report

+  $task_list = array(
+    '#theme' => 'task_list',
+    '#items' => $tasks,
+    '#active' => $active
+  );
+
+  drupal_add_region_content('sidebar_first', drupal_render($task_list));

Neither does this.

-    print theme('maintenance_page', array('content' => $status_report));
+    $maintenance_page = array(
+      '#theme' => 'maintenance_page',
+      '#content' => $status_report,
+    );
+    print drupal_render($maintenance_page);
     exit();
   }
 }
@@ -544,6 +564,11 @@ function update_check_requirements($skip_warnings = FALSE) {
   }
   else {
     drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
-    print theme('maintenance_page', array('content' => $output, 'show_messages' => !$progress_page));
+    $maintenance_page = array(
+      '#theme' => 'maintenance_page',
+      '#content' => $output,
+      '#show_messages' => !$progress_page
+    );
+    print drupal_render($maintenance_page);
   }

Or this.

The status_report() theme hook takes a render element as its only argument. This element is passed in $variables['requirements']. See system_theme().
When using this theme hook in a render array #theme property, the render structure should not have a #requirements property. It should have an array of requirements added.

Just try something like:
$status_report = array('#theme' => 'status_report') + $requirements;

I also edited my previous comment: it should have said $variables['requirements'] not $variables['#requirements'].

Indeed, it appears that this theme function is not up to the task. Thanks for sticking with this!

So what this needs now is a patch with a test that renders the example from #5 and a patch that contains both the test and the fix but not the example. @tsphethean, would you give it a spin? Need help?

The fail in Drupal\field\Tests\FieldAttachStorageTest::testFieldAttachSaveEmptyDataDefaultValue is a new one, I guess?

Anyway, the more I think about it the more I wonder why theme_status_report() declares a render element instead of a variable. When you look at the code it's not operating on a render element, but on an array of arrays with nothing but plain strings. (drupal_render() would choke on that.) Except that it checks for a '#type' property on the children. (Why?)
Perhaps we should open an issue to convert it to accept a variable. A straight conversion in system_theme() would probably work right away. (I don't think any caller in core is passing in #type properties. And it's not documented at all.)
Then again, I have no knowledge whatsoever of the history of this function.

The Twig conversion is not going to help us much, I think. I've opened #2041283: theme_status_report() is severely broken.
We should probably just work on both issues. Whichever gets in will unblock the conversion to render elements.

I'm for the approach in #2041283: theme_status_report() is severely broken

Perhaps close this as duplicate?

#2041283: theme_status_report() is severely broken got in, which makes the old patches in #2009688: Replace theme() with drupal_render() in update system and #2009674: Replace theme() with drupal_render() in system module suddenly look a lot better :-)
With the decision made to change this theme hook from taking a render element to taking a variable with plain data, the approach here is no longer needed. I'm setting this to closed (duplicate), then.

Credits to @tsphethean who really helped a lot making the theme_status_report() issue getting the attention it needed.