[[ https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_31/languages/Language.php$3413 | Language::listToText() ]] uses for(;;) which assumes the index in a list starts at 0 and following keys are consecutive integers.

This assumption fails (and results in warnings with E_STRICT) when [[ https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_31/includes/specials/SpecialListgrouprights.php$270 | SpecialListGroupRights::formatPermissions() ]] uses the key-preserving array_intersect() to create a subset of $allGroups.

Solutions include using array_values() (again) in SpecialListGroupRights::formatPermissions() or to change Language::listToText() so that it uses array_values() or foreach()

(I prefer the latter.)