20 General utilities library [utilities]

20.18 Execution policies [execpol]

20.18.5 Parallel execution policy [execpol.par]

class execution::parallel_policy { unspecified };
The class execution​::​parallel_­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 parallelized.
During the execution of a parallel algorithm with the execution​::​parallel_­policy policy, if the invocation of an element access function exits via an exception, terminate is invoked ([except.terminate]).