It would be nice to have a note about\Locale::canonicalize() within theLocale constraint documentation, since many times locales are expressed using other "uncanonicalized" formats. By instance, a common use case is to infer the locale from javascript, wherenavigator.language returns values like "en-US" instead of "en_US"; making theLocale constraint to fail.
The canonicalization process is explained atICU project.