upgrade_status-8.x-2.11/tests/modules/upgrade_status_test_theme_functions/upgrade_status_test_theme_functions.module

<?php

/**
 * Implements hook_theme().
 */
function upgrade_status_test_theme_functions_theme() {
  return [
    'upgrade_status_test_theme_function' => [
      'function' => 'upgrade_status_test_theme_function'
    ],
    'upgrade_status_test_theme_function_another_function' => [],
    'upgrade_status_test_theme_function_theme_function_override' => [],
  ];
}

/**
 * Implements hook_theme_registry_alter().
 */
function upgrade_status_test_theme_functions_theme_registry_alter(&$theme_registry) {
  $theme_registry['upgrade_status_test_theme_function_another_function']['function'] = 'upgrade_status_test_theme_function';
  $theme_registry['upgrade_status_test_theme_function_non_existing_function']['function'] = sprintf('upgrade_status_test_theme_function');
}

/**
 * Theme function used for testing.
 */
function upgrade_status_test_theme_function() {
  return 'kitten';
}