While working on SERVER-53035 unittest::ThreadAssertionMonitor, it was found that stdx::condition_variable would segfault or hit invariants where std::condition would succeed reliably.

The segfaults and invariants were observed in the SharedFuture concurrency tests, which are the first clients of the ThreadAssertionMonitor.

There's a note in thread_assertion_monitor.h, next to the _cv variable about it.
Changing _cv back to stdx::condition_variable for experimentation should reproduce the bug. It was readily observable.

It's also possible that ThreadAssertionMonitor's use of _cv is incorrect somehow and we haven't thought about it hard enough to see how.