enforce an argument to be known at compile time

constexpr_arg.cpp

#include <iostream>

template <typename T>
struct constexpr_arg
{   
    T value;
    consteval constexpr_arg(T obj) : value{obj} {}
    operator T() { return value; }
};

int mul(constexpr_arg<int> a, int b)
{
    return a*b;
}

int main()
{
    int b{3};
    std::cout << mul(5, b) << '\n'; //OK
    std::cout << mul(b, b) << '\n'; //error
}