diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 65d2afe..5ed0af6 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -637,14 +637,6 @@ protected function mergePreprocessFunctions($hook, $complete_hook, array &$cache
       $preprocess_functions = $cache[$complete_hook]['preprocess functions'];
     }
 
-    // If the hook we are completing exists, preserve it's data.
-    if (isset($cache[$hook])) {
-      $cache[$hook] = array_merge($cache[$complete_hook], $cache[$hook]);
-    }
-    else {
-      $cache[$hook] = $cache[$complete_hook];
-    }
-
     $cache[$hook]['preprocess functions'] = $preprocess_functions;
     unset($cache[$hook]['incomplete preprocess functions']);
   }
@@ -728,7 +720,9 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
       // from a pattern. This is typically set from
       // drupal_find_theme_functions() and drupal_find_theme_templates().
       if (isset($info['incomplete preprocess functions'])) {
-        // Handle manually set base hooks and suggestion hooks differently.
+        // Suggestion hooks inherit from their preceding suggestion hooks first
+        // whereas hooks with a manually set "base hook" key can just be
+        // completed based on the hook given.
         if (strpos($hook, '__')) {
           $this->completeSuggestion($hook, $cache);
         }
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index f3eb364..9caa72e 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -5,25 +5,6 @@
  * Contains \Drupal\Tests\Core\Theme\RegistryTest.
  */
 
-namespace Drupal\Core\Theme {
-
-  /**
-   * Override get_defined_functions with a configurable mock.
-   *
-   * Registry::getPrefixGroupedUserFunctions calls get_defined_functions and
-   * that is overridden here to allow testing postProcessExtension.
-   */
-  function get_defined_functions() {
-    if (\Drupal\Tests\Core\Theme\RegistryTest::$functions) {
-      return \Drupal\Tests\Core\Theme\RegistryTest::$functions;
-    }
-    else {
-      return \get_defined_functions();
-    }
-  }
-
-}
-
 namespace Drupal\Tests\Core\Theme {
 
   use Drupal\Core\Theme\ActiveTheme;
@@ -312,3 +293,22 @@ protected function initializeTheme() {
   }
 
 }
+
+namespace Drupal\Core\Theme {
+
+  /**
+   * Override get_defined_functions with a configurable mock.
+   *
+   * Registry::getPrefixGroupedUserFunctions calls get_defined_functions and
+   * that is overridden here to allow testing postProcessExtension.
+   */
+  function get_defined_functions() {
+    if (\Drupal\Tests\Core\Theme\RegistryTest::$functions) {
+      return \Drupal\Tests\Core\Theme\RegistryTest::$functions;
+    }
+    else {
+      return \get_defined_functions();
+    }
+  }
+
+}