20 General utilities library [utilities]

20.18 Execution policies [execpol]

20.18.7 Unsequenced execution policy [execpol.unseq]

class execution::unsequenced_policy { unspecified };
The class unsequenced_­policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm's execution may be vectorized, e.g., executed on a single thread using instructions that operate on multiple data items.
During the execution of a parallel algorithm with the execution​::​unsequenced_­policy policy, if the invocation of an element access function exits via an uncaught exception, terminate() is called.