22 General utilities library [utilities]

22.12 Execution policies [execpol]

22.12.3 Execution policy type trait [execpol.type]

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> is a Cpp17UnaryTypeTrait with a base characteristic of true_type if T is the type of a standard or implementation-defined execution policy, otherwise false_type.
[Note 1: 
This provision reserves the privilege of creating non-standard execution policies to the library implementation.
— end note]
The behavior of a program that adds specializations for is_execution_policy is undefined.