Overloading a function using templates

In addition to non-C++20 answer, if you are, by any chance, able to use C++20 and its concepts feature, I would suggest you the following implementation:

#include <iostream>
#include <concepts>

enum class MyEnum {
    A,
    B,
    C
};

template <typename T>
concept IntegralOrEnum = std::same_as<MyEnum, T> || std::integral<T>;

template <IntegralOrEnum T>
bool isFunction(T const& aVariable) {
    return true;
}

int main() {
    isFunction(MyEnum::A);
    isFunction(3);
    isFunction("my_string"); // error
    return 0;
}

Demo

UPDATE

According to @RichardSmith’s comment, here is a more scalable and reusable approach:

template <typename T, typename ...U>
concept one_of = (std::is_same_v<T, U> || ...);

template <one_of<int, MyEnum> T>
bool isFunction(T const& aVariable) {
    return true;
}