Function is_cross_guild 

fn is_cross_guild(link: GuildId, source: GuildId) -> bool

Returns true when a link crosses a guild boundary.

Roles, permission overwrites and the @everyone id are all guild-local, so the role-set comparison in check_visibility cannot judge a channel in another guild — and the bot may not even be a member of that guild. Such links are refused outright rather than judged.