Issue #2493989 by Antti J. Salminen, lauriii, slashrsm: template_preprocess() and other preprocess functions are called even for theme hooks implemented as functions

core/lib/Drupal/Core/Theme/Registry.php

@@ -472,13 +472,15 @@ class Registry implements DestructableInterface {
         // A template file is the default implementation for a theme hook, but
         // if the theme hook specifies a function callback instead, check to
         // ensure the function actually exists.
-        if (isset($info['function']) && !function_exists($info['function'])) {
+        if (isset($info['function'])) {
+          if (!function_exists($info['function'])) {
             throw new \BadFunctionCallException(sprintf(
               'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
               $hook,
               $info['function']
             ));
           }
+        }
         // Provide a default naming convention for 'template' based on the
         // hook used. If the template does not exist, the theme engine used
         // should throw an exception at runtime when attempting to include

core/modules/system/src/Tests/Theme/RegistryTest.php

@@ -100,6 +100,11 @@ class RegistryTest extends KernelTestBase {
       'test_basetheme_preprocess_theme_test_template_test',
     ], $preprocess_functions);
 
+    $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
+    $this->assertIdentical([
+       'template_preprocess_theme_test_function_suggestions',
+       'test_basetheme_preprocess_theme_test_function_suggestions',
+    ], $preprocess_functions, "Theme functions don't have template_preprocess but do have template_preprocess_HOOK");
   }
 
   /**

core/modules/system/tests/modules/theme_test/theme_test.module

@@ -81,6 +81,12 @@ function theme_test_page_bottom(array &$page_bottom) {
   $page_bottom['theme_test_page_bottom'] = array('#markup' => 'theme test page bottom markup');
 }
 
+/**
+ * Implements template_preprocess_HOOK() for theme_test_function_suggestions theme functions.
+ */
+function template_preprocess_theme_test_function_suggestions(&$variables) {
+}
+
 /**
  * Theme function for testing _theme('theme_test_foo').
  */

core/modules/system/tests/themes/test_basetheme/test_basetheme.theme

@@ -28,3 +28,9 @@ function test_basetheme_views_post_render(ViewExecutable $view, &$output, CacheP
  */
 function test_basetheme_preprocess_theme_test_template_test(&$variables) {
 }
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_test_function_suggestions theme functions.
+ */
+function test_basetheme_preprocess_theme_test_function_suggestions(&$variables) {
+}