numeric_cast should work with enum's

From: Frederick Virchanza Gotham (cauldwell.thomas_at_[hidden])
Date: 2022-03-16 15:12:07

(Sometimes Gmail takes out white space so I'm hoping this post is
indented properly.)

I think that "numeric_cast" should be able to cast to and from enum
types, so that it can be used as follows:

#include <cstdint>
#include <iostream>
using std::cout;
using std::endl;

enum class Toad : std::uint8_t { Purple, Orange = 0x7f };

enum class Frog { Blue, Yellow, Pink = 0xFFFFu };

int main(void)
{
    try { numeric_cast<Toad>( Frog::Pink ); } catch(std::exception
const &e) { cout << e.what() << endl; }
    std::uint32_t my_32_bit_var = 0xffffffffU;

    try { numeric_cast<std::uint16_t>(my_32_bit_var); }
catch(std::exception const &e) { cout << e.what() << endl; }
    try { numeric_cast<std::uint8_t>( Frog::Pink ); }
catch(std::exception const &e) { cout << e.what() << endl; }
    try { numeric_cast<Toad>(0x1FFu); } catch(std::exception const &e)
{ cout << e.what() << endl; }
}

In order to achieve this behaviour, I've created my own "numeric_cast"
in the global namespace as follows:

#include <type_traits>                        // is_enum, underlying_type
#include <boost/numeric/conversion/cast.hpp>  // numeric_cast

template<bool T_is_an_enum, typename T>
struct numeric_cast_helper {
    typedef T type;
};

template<typename T>
struct numeric_cast_helper<true, T> {
    typedef typename std::underlying_type<T>::type type;
};

template<typename A, typename B>
A numeric_cast(B const b)  // This function takes its argument by
value, and also returns by value -- which is fine for arithmetic types
{
    return
    static_cast<A>(
        boost::numeric_cast<typename
numeric_cast_helper<std::is_enum<A>::value, A>::type>
        (
            static_cast<typename
numeric_cast_helper<std::is_enum<B>::value, B>::type>(b)
        )
    );
}