std::shared_mutex and boost::shared_mutex are disallowed but no stdx:: exists

Lint complains that std::shared_lock (and boost::shared_lock) should not be used, in favor of stdx::shared_lock:

Error: src/mongo/db/storage/storage_engine_change_context.h:54 - mongodb/polyfill - Illegal use of banned name from std::/boost:: for "std::shared_lock", use mongo::stdx:: variant instead

But there is no stdx::shared_lock
In file included from src/mongo/db/storage/storage_engine_init.cpp:45:
src/mongo/db/storage/storage_engine_change_context.h:76:22: error: no member named 'shared_lock' in namespace 'mongo::stdx'
return stdx::shared_lock(_storageChangeSpinlock);