template<class T> struct is_execution_policy { see below };
is_execution_policy can be used to detect execution policies for the purpose of excluding function signatures from otherwise ambiguous overload resolution participation.
is_execution_policy<T> shall be a UnaryTypeTrait with a base characteristic of true_type if T is the type of a standard or implementation-defined execution policy, otherwise false_type.
[ Note: This provision reserves the privilege of creating non-standard execution policies to the library implementation. — end note ]