.. change::
    :tags: feature, region
    :tickets: 236

    Added new construct :class:`.api.CantDeserializeException` which can be
    raised by user-defined deserializer functions which would be passed to
    :paramref:`.CacheRegion.deserializer`, to indicate a cache value that can't
    be deserialized and therefore should be regenerated. This can allow an
    application that's been updated to gracefully re-cache old items that were
    persisted from a previous version of the application. Pull request courtesy
    Simon Hewitt.