std::unexpect_t, std::unexpect

Defined in header `<expected>`
`struct unexpect_t { explicit unexpect_t() = default; };`	(1)	(since C++23)
`inline constexpr unexpect_t unexpect{};`	(2)	(since C++23)

1) A tag type for in-place construction of an unexpected value in an std::expected object.

2) A constant of type const std::unexpect_t which is usually directly passed to a constructor of std::expected to construct an unexpected value.

Like other construction tag types, unexpect_t is a trivial, empty class with an explicit default constructor.

(constructor)	constructs the `expected` object (public member function)
in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t (C++17)	in-place construction tag (tag)

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