The following is paraphrased from an issue raised by Andrzej Krzemieński as a github issue. It touches upon fundamental ideas behind the library and how these ideas as the implementation of the library collided with reality.

“In the current implementation safe<T> will only work with T being a C++ scalar type. Therefore making a general type requirements that say what operations are allowed is superfluous, and confusing (because it implies that safe<> is more generic.”

When I started out, It became clear that I wanted "safe" types to look like "numeric" types. It also became clear pretty soon that there was going to be significant template meta-programming in the implementation. Normal type traits like std::is_integer are defined in the std namespace and one is discouraged from extending it. Also I needed some compile time "max" and "lowest" values. This lead me to base the design on std::numeric_limits. But std::numeric limits is inherently extensible to any "numeric" type. For example, money is a numeric type but not an intrinsic types. So it seemed that I needed to define a "numeric" concept which required that there be an implementation of std::numeric_limits for any type T - such as money in this case. When I'm doubt - I tend to think big.

For now though I'm not going to address it. For what it's worth, my preference would be to do something like:

template<typename T>
struct range {
    T m_lowest;
    T m_highest;
    // default implementation
    range(
        const & T t_min, 
        const & T t_max
    ) :
        m_lowest(std::numeric_limits<T>::lowest(t_min),
        m_highest(std::numeric_limits<T>::max(t_max)
    {}
};

Then redeclare safe_base, etc., accordingly.

Also not that for C++20, template value parameters are no longer restricted to integer primitive types buy maybe class types as well. This means the library maybe extended to user class types without changing the current template signatures.

The following is paraphrased from an issue raised by Andrzej Krzemieński as a github issue.

“You do not need a concept to constrain anything with it, in your library. Or is the purpose of the Type requirements to show in detail what it means that safe<T> is a 'drop-in replacement for T?”

Right - currently I don't use the concept to constrain anything. They are currently a purely "conceptual" tool to keep the design from getting off track. This is common with other libraries such as the C++ standard library where the concepts are defined but not enforced by compile time predicates. Hopefully in future that might change - see below

“If you want to extend safe<T> for other integer types, Type requirement still need to be fixed:”

Hmmmm - I'm not quite sure that this is true. One thing that IS true is the the interface and implementation of the library will need to be enhanced to permit "safe" to be applied to user defined types. This is apparent now, but as my brain can only comprehend the library one piece at a time, this design feature was lost during the implementation. In implementing co-existence of floats with safe integers, I did refactor the implementation in a way which I believe my eventually permit the application to any user supplied T which implements all the required operations of Numeric types. So as it is now this is pending. If the library were to become widely used, there might be motivation to do this. Time will tell. So for now I'm leaving these in the documentation and code, even though they are not actually used.