tmp/tmp1u_te13p/{from.md → to.md}
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### Semiregular wrapper <a id="range.semi.wrap">[[range.semi.wrap]]</a>
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Many types in this subclause are specified in terms of an
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exposition-only class template *`semiregular-box`*. `semiregular-box<T>`
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behaves exactly like `optional<T>` with the following differences:
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- `semiregular-box<T>` constrains its type parameter `T` with
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`copy_constructible<T> && is_object_v<T>`.
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- If `T` models `default_initializable`, the default constructor of
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`semiregular-box<T>` is equivalent to:
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``` cpp
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constexpr semiregular-box() noexcept(is_nothrow_default_constructible_v<T>)
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: semiregular-box{in_place}
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{ }
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```
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- If `assignable_from<T&, const T&>` is not modeled, the copy assignment
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operator is equivalent to:
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``` cpp
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semiregular-box& operator=(const semiregular-box& that)
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noexcept(is_nothrow_copy_constructible_v<T>)
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{
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if (that) emplace(*that);
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else reset();
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return *this;
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}
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```
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- If `assignable_from<T&, T>` is not modeled, the move assignment
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operator is equivalent to:
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``` cpp
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semiregular-box& operator=(semiregular-box&& that)
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noexcept(is_nothrow_move_constructible_v<T>)
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{
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if (that) emplace(std::move(*that));
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else reset();
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return *this;
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}
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```
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