https://docs.mongodb.com/manual/reference/configuration-options/#storage.repairPath states:

Not available for mongod instances that use the in-memory storage engine.

However MongoDB 3.4 with WiredTiger produces this error message when run with the repairpath option:

[initandlisten] exception in initAndListen: 40311 Cannot start server. The command line option '--repairpath' is only supported by the mmapv1 storage engine, terminating 

This behaviour was added in MongoDB 3.3.14 as per SERVER-25008. Based on HELP-2295, repairpath has always been ignored by storage engines other than MMAPv1, but previously we silently ignored the option.