diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 7c9f4d1..6b6fd86 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -566,6 +566,16 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
           $info['preprocess functions'] = array_merge($cache[$hook]['preprocess functions'], $info['preprocess functions']);
         }
         $result[$hook]['preprocess functions'] = $info['preprocess functions'];
+
+        // Before merging the new result with the original information, ensure
+        // that the final result does not end up with both a template and a
+        // function defined.
+        if (isset($result[$hook]['function'])) {
+          unset($cache[$hook]['template']);
+        }
+        elseif (isset($result[$hook]['template'])) {
+          unset($cache[$hook]['function']);
+        }
       }
 
       // Merge the newly created theme hooks into the existing cache.