Standard library header <execution>

namespace std {
  // execution policy type trait
  template<class T> struct is_execution_policy;
  template<class T>
  inline constexpr bool is_execution_policy_v = is_execution_policy<T>::value;
}
 
namespace std::execution {
  // sequenced execution policy
  class sequenced_policy;
 
  // parallel execution policy
  class parallel_policy;
 
  // parallel and unsequenced execution policy
  class parallel_unsequenced_policy;
 
  // unsequenced execution policy
  class unsequenced_policy;
 
  // execution policy objects
  inline constexpr sequenced_policy            seq{ /*unspecified*/ };
  inline constexpr parallel_policy             par{ /*unspecified*/ };
  inline constexpr parallel_unsequenced_policy par_unseq{ /*unspecified*/ };
  inline constexpr unsequenced_policy          unseq{ /*unspecified*/ };
}