How to create a `std::integer_sequence` out of `__func__`?

Question

I'm trying to create a function foobar() that can be used as...

int main()
{
    auto x = foobar(__func__);
    // decltype(x) = std::integer_sequence<char, 'm', 'a', 'i', 'n'>
}

Any hints?

The code does not need to work on MSVC.

Answer 1

Here is the approach that works with GCC and Clang without need for C++20 using the statement expression language extension:

template<std::size_t n>
struct helper {
    const char(& s)[n];
    
    constexpr helper(const char(& str)[n]) : s(str)
    {}
    
    template<class Fn, std::size_t... is>
    static constexpr auto apply(Fn fn, std::index_sequence<is...>) {
        return fn(std::integral_constant<std::size_t, is>{}...);
    }

    template<class Fn>
    static constexpr auto apply(Fn fn) {
        return apply(fn, std::make_index_sequence<n - 1>{});
    }
};


#define arr_as_char_sequence(ARR)                                         \
    ({ constexpr helper my_helper(ARR); my_helper.apply([](auto... is)    \
          { return std::integer_sequence<char, my_helper.s[is]...>{}; }); });

void foo() {
    auto name = arr_as_char_sequence(__func__);
    static_assert(std::is_same_v<
        decltype(name), std::integer_sequence<char, 'f', 'o', 'o'>>);
}

Demo