Temporary files not stored in temporary directory

On fresh Drupal-HEAD (#81468), Drupal doesn't take into account the 'temporary files directory' as set on the 'file system' configuration page.

Instead, all files (including temporary files) are just put in the /files folder directly.

A quick look at the code takes me to file_save_upload with this snippet of code:

<?php
if (!$dest) {
      $dest = file_destination(file_create_path($file->filename), FILE_EXISTS_RENAME);
    }
?>

... but file_create_path is expecting its argument to be the path, not just the file name.

Presumably caused by rolling file_check_upload into file_save_upload.