Template:cpp/experimental/execution policy note


 * If is an instance of, all operations are performed in the calling thread.
 * If is an instance of, operations may be performed in unspecified number of threads, indeterminately sequenced with each other
 * If is an instance of, execution may be both parallelized and vectorized: function body boundaries are not respected and user code may be overlapped and combined in arbitrary manner (in particular, this implies that a user-provided Callable must not acquire a mutex to access a shared resource)