33 Concurrency support library [thread]

33.4 Threads [thread.threads]

33.4.3 Class thread [thread.thread.class] Destructor [thread.thread.destr]

Effects: If joinable(), invokes terminate ([except.terminate]).
Otherwise, has no effects.
[Note 1: 
Either implicitly detaching or joining a joinable() thread in its destructor can result in difficult to debug correctness (for detach) or performance (for join) bugs encountered only when an exception is thrown.
These bugs can be avoided by ensuring that the destructor is never executed while the thread is still joinable.
— end note]