[expected.unexpected]

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namespace std { template<class E> class unexpected { public: / [expected.un.cons], constructors constexpr unexpected(const unexpected&) = default; constexpr unexpected(unexpected&&) = default; template<class Err = E> constexpr explicit unexpected(Err&&); template<class. Args> constexpr explicit unexpected(in_place_t, Args&&.); template<class U, class. Args> constexpr explicit unexpected(in_place_t, initializer_list<U>, Args&&.); constexpr unexpected& operator=(const unexpected&) = default; constexpr unexpected& operator=(unexpected&&) = default; constexpr const E& error() const & noexcept; constexpr E& error() & noexcept; constexpr const E&& error() const && noexcept; constexpr E&& error() && noexcept; constexpr void swap(unexpected& other) noexcept(see below); template<class E2> friend constexpr bool operator=(const unexpected&, const unexpected<E2>&); friend constexpr void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y)); private: E unex; / exposition only }; template<class E> unexpected(E) -> unexpected<E>; }

A program that instantiates the definition of unexpected for a non-object type, an array type, a specialization of unexpected, or a cv-qualified type is ill-formed.

22.8.3.2 Constructors [expected.un.cons]

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template<class Err = E>
  constexpr explicit unexpected(Err&& e);

Throws: Any exception thrown by the initialization of unex.

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template<class. Args>
  constexpr explicit unexpected(in_place_t, Args&&. args);

Effects: Direct-non-list-initializes unex with std::forward<Args>(args)..

Throws: Any exception thrown by the initialization of unex.

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template<class U, class. Args>
  constexpr explicit unexpected(in_place_t, initializer_list<U> il, Args&&. args);

Effects: Direct-non-list-initializes unex with il, std::forward<Args>(args)..

Throws: Any exception thrown by the initialization of unex.

22.8.3.3 Observers [expected.un.obs]

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constexpr const E& error() const & noexcept;
constexpr E& error() & noexcept;

Returns: unex.

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constexpr E&& error() && noexcept;
constexpr const E&& error() const && noexcept;

Returns: std::move(unex).

22.8.3.4 Swap [expected.un.swap]

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constexpr void swap(unexpected& other) noexcept(is_nothrow_swappable_v<E>);

Effects: Equivalent to: using std::swap; swap(unex, other.unex);

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friend constexpr void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y));

Effects: Equivalent to x.swap(y).

22.8.3.5 Equality operator [expected.un.eq]

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template<class E2>
  friend constexpr bool operator=(const unexpected& x, const unexpected<E2>& y);

Mandates: The expression x.error() == y.error() is well-formed and its result is convertible to bool.

Returns: x.error() == y.error().