Source/WTF/wtf/Unexpected.h

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// Implementation of Library Fundamentals v3's std::expected, as described here: http://wg21.link/p0323r4

#pragma once

/*
    unexpected synopsis

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {
    // ?.?.3, Unexpected object type
    template <class E>
      class unexpected;

    // ?.?.4, Unexpected relational operators
    template <class E>
        constexpr bool
        operator==(const unexpected<E>&, const unexpected<E>&);
    template <class E>
        constexpr bool
        operator!=(const unexpected<E>&, const unexpected<E>&);

    template <class E>
    class unexpected {
    public:
        unexpected() = delete;
        template <class U = E>
          constexpr explicit unexpected(E&&);
        constexpr const E& value() const &;
        constexpr E& value() &;
        constexpr E&& value() &&;
        constexpr E const&& value() const&&;
    private:
        E val; // exposition only
    };

}}}

*/

#include <cstdlib>
#include <utility>
#include <wtf/StdLibExtras.h>

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {

template<class E>
class unexpected {
    WTF_MAKE_FAST_ALLOCATED;
public:
    unexpected() = delete;
    template <class U = E>
    constexpr explicit unexpected(U&& u) : val(std::forward<U>(u)) { }
    constexpr const E& value() const & { return val; }
    constexpr E& value() & { return val; }
    constexpr E&& value() && { return WTFMove(val); }
    constexpr const E&& value() const && { return WTFMove(val); }

private:
    E val;
};

template<class E> constexpr bool operator==(const unexpected<E>& lhs, const unexpected<E>& rhs) { return lhs.value() == rhs.value(); }
template<class E> constexpr bool operator!=(const unexpected<E>& lhs, const unexpected<E>& rhs) { return lhs.value() != rhs.value(); }

}}} // namespace std::experimental::fundamentals_v3

template<class E> using Unexpected = std::experimental::unexpected<E>;

// Not in the std::expected spec, but useful to work around lack of C++17 deduction guides.
template<class E> constexpr Unexpected<std::decay_t<E>> makeUnexpected(E&& v) { return Unexpected<typename std::decay<E>::type>(std::forward<E>(v)); }