[Bug c++/100157] Support `__type_pack_element` like Clang

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100157

m.cencora at gmail dot com changed:

           What    |Removed                     |Added
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                 CC|                            |m.cencora at gmail dot com

--- Comment #2 from m.cencora at gmail dot com ---
Please consider also adding a builtin for fetching index of type in type list.
While there are some easy implementations (like following), they are still not
very performant.

template <typename T>
constexpr auto uniqId()
{
    return __PRETTY_FUNCTION__;
}

template <typename U, unsigned N>
constexpr unsigned index_of_impl(U needle, const U (&haystack)[N])
{
    for (unsigned i = 0u; i != N; ++i)
    {
        if (haystack[i] == needle)
        {
            return i;
        }
    }
    return -1;
}

template <typename T, typename ...Ts>
constexpr auto index_of_v = index_of_impl(uniqId<T>, { uniqId<Ts>...});

using T1 = float;
using T2 = int;
struct A;
struct B;

auto c = index_of_v<A, T2, B, A, T1>;