std::is_constant_evaluated Library in C++20

The function std::is_constant_evaluted determines whether the function is executed at compile time or run time. Why do we need this function from the type-traits library? In C++20, we have roughly spoken of three kinds of functions:

• consteval declared functions run at compile time: consteval int alwaysCompiletime();

• constexpr declared functions can run at compile time or run time: constexpr int ...