What should moving from `optional<T&>` into `optional<T>` do?

[jiixyj]

I'm wondering about the interactions between optional<T&> and optional<T>. Specifically about optional<T>'s "converting" move constructor when the RHS is a optional<T&>. Should there be a move from the "remote" value, or should the value category be shallow, leading to a copy? Example:

    {
        std::string s = "qwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnm";
        beman::optional26::optional<std::string&> o1 = s;
        beman::optional26::optional<std::string> o2 = std::move(o1);
        EXPECT_???(s.empty());
    }

At the moment, there is a move happening, and s is empty. I'm not sure if this is intended behavior though, or a "happy accident".

In any case, the "converting" move constructor of optional<T> seems not yet aware of optional<T&>:

/// Converting move constructor.
template <class T>
template <class U>
inline constexpr optional<T>::optional(optional<U>&& rhs)
    requires detail::enable_from_other<T, U, U&&> && std::is_convertible_v<U&&, T>
{
    if (rhs.has_value()) {
        construct(std::move(*rhs));
    }
}

...if U is a reference here, the U&&'s collapse to simple references. So detail::enable_from_other actually checks if it can copy the value of the rhs in the constraints, but then does a move inside the function body.

[jiixyj]

FWIW, std::reference_wrapper does a copy in this case, not a move:

    {
        std::string s = "qwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnm";
        std::reference_wrapper<std::string> o1 = s;
        std::string o2 = std::move(o1);
        EXPECT_FALSE(s.empty());
    }

[jiixyj]

I've opened a PR with a possible fix: #93

[jiixyj]

Regarding the standard, I think in the following two places std::move(*rhs) should read *std::move(rhs):

• https://eel.is/c++draft/optional#ctor-34
• https://eel.is/c++draft/optional#assign-25