MDL-42553 core_calendar: third party calendar types can now be uninst…

…alled

lib/classes/plugininfo/calendartype.php

@@ -26,11 +26,18 @@
 defined('MOODLE_INTERNAL') || die();
 
 /**
- * Class for admin tool plugins
+ * Class for calendar type plugins.
  */
 class calendartype extends base {
 
     public function is_uninstall_allowed() {
+        // We can delete all calendar types, except Gregorian. Gregorian comes with core and was the calendar
+        // type used before the calendar types were introduced as plugins in Moodle. If all calendar types were
+        // deleted then Moodle would break completely wherever any dates are displayed.
+        if ($this->name !== 'gregorian') {
+            return true;
+        }
+
         return false;
     }
 }