MDL-81683 core_courseformat: Prevent subsection to be moved onto itself

* Subsections are considered as section and need to be prevented to
be moved into itself so to avoid infinite recursive structures.

course/format/classes/stateactions.php

@@ -85,6 +85,10 @@ public function cm_move(
             $cm = $modinfo->get_cm($cmid);
             $currentsectionid = $cm->section;
             $targetsection = $modinfo->get_section_info_by_id($targetsectionid, MUST_EXIST);
+            if ($targetsection->is_delegated() && $cm->get_delegated_section_info()) {
+                throw new moodle_exception('subsectionmoveerror', 'core', '', null, 'Cannot move section on' .
+                    ' another subsection');
+            }
             $beforecm = (!empty($beforecmdid)) ? $modinfo->get_cm($beforecmdid) : null;
             if ($beforecm === null || $beforecm->id != $cmid) {
                 moveto_module($cm, $targetsection, $beforecm);

lang/en/moodle.php

@@ -1982,6 +1982,7 @@
 $string['secs'] = 'secs';
 $string['section'] = 'Section';
 $string['sectionactionnotsupported'] = 'Section action "{$a}" is not supported here';
+$string['subsectionmoveerror'] = 'Subsection moved here';
 $string['sectionname'] = 'Section name';
 $string['sectionoutline'] = 'Section outline';
 $string['sections'] = 'Sections';