diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 6b6fd86..7cf4b3c 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -567,13 +567,11 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
         }
         $result[$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'])) {
+        // 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']);
         }
       }