Issue #2861840 by tim.plunkett, lauriii: Preprocess functions are not merged when a module registers a theme hook for a theme-provided template

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

@@ -2,6 +2,7 @@
 
 namespace Drupal\Core\Theme;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\DestructableInterface;
@@ -565,10 +566,18 @@ class Registry implements DestructableInterface {
           $info['preprocess functions'] = array_merge($cache[$hook]['preprocess functions'], $info['preprocess functions']);
         }
         $result[$hook]['preprocess functions'] = $info['preprocess functions'];
+
+        // If a theme implementation definition provides both 'template' and
+        // 'function', the 'function' will be used. In this case, if the new
+        // result provides a 'template' value, any existing 'function' value
+        // must be removed for the override to be called.
+        if (isset($result[$hook]['template'])) {
+          unset($cache[$hook]['function']);
+        }
       }
 
       // Merge the newly created theme hooks into the existing cache.
-      $cache = $result + $cache;
+      $cache = NestedArray::mergeDeep($cache, $result);
     }
 
     // Let themes have variable preprocessors even if they didn't register a

core/modules/layout_discovery/tests/src/Kernel/LayoutTest.php

@@ -33,6 +33,17 @@ class LayoutTest extends KernelTestBase {
     $this->layoutPluginManager = $this->container->get('plugin.manager.core.layout');
   }
 
+  /**
+   * Tests that a layout provided by a theme has the preprocess function set.
+   */
+  public function testThemeProvidedLayout() {
+    $this->container->get('theme_installer')->install(['test_layout_theme']);
+    $this->config('system.theme')->set('default', 'test_layout_theme')->save();
+
+    $theme_definitions = $this->container->get('theme.registry')->get();
+    $this->assertTrue(in_array('template_preprocess_layout', $theme_definitions['test_layout_theme']['preprocess functions']));
+  }
+
   /**
    * Test rendering a layout.
    *

core/modules/layout_discovery/tests/themes/test_layout_theme/templates/test-layout-theme.html.twig

@@ -0,0 +1 @@
+{{ content.content }}

core/modules/layout_discovery/tests/themes/test_layout_theme/test_layout_theme.info.yml

@@ -0,0 +1,6 @@
+name: 'Test layout theme'
+type: theme
+description: 'Theme for testing a theme-provided layout'
+version: VERSION
+base theme: classy
+core: 8.x

core/modules/layout_discovery/tests/themes/test_layout_theme/test_layout_theme.layouts.yml

@@ -0,0 +1,7 @@
+test_layout_theme:
+  label: 'Test Layout - Theme'
+  category: 'Test Layout Theme'
+  template: templates/test-layout-theme
+  regions:
+    content:
+      label: Content

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

@@ -66,6 +66,10 @@ function theme_test_theme($existing, $type, $theme, $path) {
       'bar' => '',
     ],
   ];
+  $items['theme_test_registered_by_module'] = [
+    'render element' => 'content',
+    'base hook' => 'container',
+  ];
   return $items;
 }
 
@@ -213,3 +217,9 @@ function theme_test_theme_suggestions_node(array $variables) {
 
   return $suggestions;
 }
+
+/**
+ * Implements template_preprocess_HOOK() for theme_test_registered_by_module.
+ */
+function template_preprocess_theme_test_registered_by_module() {
+}

core/modules/system/tests/themes/test_theme/templates/theme-test-registered-by-module.html.twig

@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template provided by theme is registered by module.

core/tests/Drupal/KernelTests/Core/Theme/RegistryTest.php

@@ -192,4 +192,23 @@ class RegistryTest extends KernelTestBase {
     ], $suggestions, 'Found expected page node suggestions.');
   }
 
+  /**
+   * Tests theme-provided templates that are registered by modules.
+   */
+  public function testThemeTemplatesRegisteredByModules() {
+    $theme_handler = \Drupal::service('theme_handler');
+    $theme_handler->install(['test_theme']);
+
+    $registry_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_theme');
+    $registry_theme->setThemeManager(\Drupal::theme());
+
+    $expected = [
+      'template_preprocess',
+      'template_preprocess_container',
+      'template_preprocess_theme_test_registered_by_module'
+    ];
+    $registry = $registry_theme->get();
+    $this->assertEquals($expected, array_values($registry['theme_test_registered_by_module']['preprocess functions']));
+  }
+
 }