[C++20] 60 LOC constexpr get_name for members / no ifdefs / Clang,GCC,msvc

Resources:

- https://reddit.com/r/cpp/comments/1890jr9/reflectcpp_automat... - https://www.reddit.com/r/cpp/comments/18b8iv9/c20_to_tuple_w...

Auto-tuning version of constexpr get_name for members

    struct foo {
        int bar;
        int baz;
    };

    static_assert("bar"sv == get_name<0, foo>);
    static_assert("baz"sv == get_name<1, foo>);

Basic idea:

    template <auto Ptr> [[nodiscard]] consteval auto get_name() -> std::string_view { 
        return std::source_location::current().function_name();
    }
    template <class T> extern const T external; // magic happens here
    constexpr auto ptr(auto&& t) { auto&& [p1] = t; return &p1; }

    struct foo { int field; };

    std::cout << get_name<ptr(external<foo>)>(); // prints ...field...

Auto-tuning the name (to avoid ifdefs for compilers): // define a $struct$.$filed$ to find the parsing requirements for given compiler

    struct $struct$ {
        int $field$; // pick some name / $ is valid identifier now
    };
    constexpr auto $name = get_name_impl<0, struct$>;
    constexpr auto $end = $name.substr($name.find("$field$") + sizeof("$field$") - 1);
    constexpr auto $begin = $name[$name.find("$field$") - 1];

    template <auto N, class T>
    constexpr auto get_name = [] {
        const auto name = get_name_impl<N, T>;
        const auto begin = name.find(end);
        const auto tmp = name.substr(0, begin);
        return tmp.substr(tmp.find_last_of(begin) + 1);
    }();

Full example - https://godbolt.org/z/MWhf6voTs