Prevent calling make_optional<int&>(i) or make_optional<const int&>(i)

[Quuxplusone]

The status quo in C++23 is that

    int i;
    auto o = make_optional<int&>(i);

produces an optional<int>, not an optional<int&>. This would be horribly confusing to propagate into C++26 — users would naturally expect to receive an optional<int&>. Worse,

    auto o = make_optional<int&>(std::ref(i));

actually would give you an optional<int&>, because it would use a different overload of make_optional! So, we propose to add a "Mandates" element to make_optional that requires its explicit template argument (if any) to be a non-array object type.
This breaks the "status quo" example at the top of this comment (requiring an Annex C entry); but it successfully prevents the pitfalls. And it does not silently change the behavior of the "status quo" example, either — which would probably be 100x worse than merely breaking it!

Thanks to Tomasz Kamiński for the template-programming trick, which matches the technique used in https://eel.is/c++draft/out.ptr .

[frederick-vs-ja]

How about making the first overload like this, which seems simpler to me?

    namespace detail {
        struct spec_barrier {
            explicit spec_barrier() = default;
        };
    }

    template<detail::spec_barrier = detail::spec_barrier{}, class T>
        requires std::is_constructible_v<std::decay_t<T>, T> // per LWG3627
    constexpr std::optional<std::decay_t<T>> make_optional(T&& t)
        noexcept(std::is_nothrow_constructible_v<std::decay_t<T>, T>)
    {
        return std::optional<std::decay_t<T>>(std::forward<T>(t));
    }

[Quuxplusone]

How about making the first overload like this, which seems simpler to me?

Oh yeah — that works, because even though today the first overload is used for e.g. make_optional<int&>(i), there's no reason it needs to be. We can make the first overload non-viable in that case, and then the second overload will be used instead, which is fine! (Until someone points out a subtle ABI problem with that change, anyway... ;))

Updated accordingly.