Condition variable and shared mutex
This is defined by the standard to allow for maximum efficiency of the implementation. If you want to use a different lock with condition_variable you need to use condition_variable_any. Note that the condition_variable_any implementation has some overhead though.
Quote from the standard: 30.5 Condition variables
Class
condition_variableprovides a condition variable that can only wait on an object of typeunique_lock<mutex>, allowing maximum effciency on some platforms.