Issue #2002418 by Psikik: Rename Views method can_group() to canGroup().

core/modules/views/lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php

@@ -1429,7 +1429,7 @@ public function query() {
    *
    * @return bool
    */
-   function can_group() {
+   public function canGroup() {
      return TRUE;
    }
 

core/modules/views/lib/Drupal/views/Plugin/views/filter/GroupByNumeric.php

@@ -59,6 +59,6 @@ public function adminLabel($short = FALSE) {
     return $this->getField(parent::adminLabel($short));
   }
 
-  function can_group() { return FALSE; }
+  public function canGroup() { return FALSE; }
 
 }

core/modules/views_ui/lib/Drupal/views_ui/Form/Ajax/RearrangeFilter.php

@@ -142,7 +142,7 @@ public function buildForm(array $form, array &$form_state) {
       }
 
       $handler = $display->getHandler($type, $id);
-      if ($grouping && $handler && !$handler->can_group()) {
+      if ($grouping && $handler && !$handler->canGroup()) {
         $field['group'] = 'ungroupable';
       }