nonmutating’ is not defined .

- ["nonmutating"] provides backward compatibility for idempotent
    operations that were previously declared using the deprecated
    'nonmutating' keyword. You can replace the 'nonmutating' keyword
    with '["nonmutating"] idempotent' to maintain compatibility with
    objects implemented using Ice 3.0 or 3.1.

用元数据指令=["nonmutating"]替代关键字nonmutating