From Jason Turner

[optional.swap]

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  #### Swap <a id="optional.swap">[[optional.swap]]</a>
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  ``` cpp
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- void swap(optional& rhs) noexcept(see below);
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  ```
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  *Mandates:* `is_move_constructible_v<T>` is `true`.
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- *Preconditions:* Lvalues of type `T` are swappable.
 
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  *Effects:* See [[optional.swap]].
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  **Table: `optional::swap(optional&)` effects** <a id="optional.swap">[optional.swap]</a>
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  | | `*this` contains a value | `*this` does not contain a value |
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- | ------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
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- | `rhs` contains a value | calls `swap(*(*this), *rhs)` | initializes the contained value of `*this` as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`, followed by `rhs.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value |
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- | `rhs` does not contain a value | initializes the contained value of `rhs` as if direct-non-list-initializing an object of type `T` with the expression `std::move(*(*this))`, followed by `val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value | no effect |
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  *Throws:* Any exceptions thrown by the operations in the relevant part
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  of [[optional.swap]].
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- *Remarks:* The expression inside `noexcept` is equivalent to:
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  ``` cpp
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  is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>
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  ```
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- If any exception is thrown, the results of the expressions `bool(*this)`
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- and `bool(rhs)` remain unchanged. If an exception is thrown during the
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- call to function `swap`, the state of `*val` and `*rhs.val` is
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- determined by the exception safety guarantee of `swap` for lvalues of
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- `T`. If an exception is thrown during the call to `T`’s move
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- constructor, the state of `*val` and `*rhs.val` is determined by the
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- exception safety guarantee of `T`’s move constructor.
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  #### Swap <a id="optional.swap">[[optional.swap]]</a>
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  ``` cpp
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+ constexpr void swap(optional& rhs) noexcept(see below);
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  ```
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  *Mandates:* `is_move_constructible_v<T>` is `true`.
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+ *Preconditions:* `T` meets the *Cpp17Swappable*
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+ requirements [[swappable.requirements]].
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  *Effects:* See [[optional.swap]].
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  **Table: `optional::swap(optional&)` effects** <a id="optional.swap">[optional.swap]</a>
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  | | `*this` contains a value | `*this` does not contain a value |
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+ | ------------------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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+ | `rhs` contains a value | calls `swap(*(*this), *rhs)` | direct-non-list-initializes the contained value of `*this` with `std::move(*rhs)`, followed by `rhs.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value |
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+ | `rhs` does not contain a value | direct-non-list-initializes the contained value of `rhs` with `std::move(*(*this))`, followed by `val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value | no effect |
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  *Throws:* Any exceptions thrown by the operations in the relevant part
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  of [[optional.swap]].
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+ *Remarks:* The exception specification is equivalent to:
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  ``` cpp
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  is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>
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  ```
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+ If any exception is thrown, the results of the expressions
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+ `this->has_value()` and `rhs.has_value()` remain unchanged. If an
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+ exception is thrown during the call to function `swap`, the state of
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+ `*val` and `*rhs.val` is determined by the exception safety guarantee of
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+ `swap` for lvalues of `T`. If an exception is thrown during the call to
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+ `T`’s move constructor, the state of `*val` and `*rhs.val` is determined
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+ by the exception safety guarantee of `T`’s move constructor.
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