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### 2748. `swappable` traits for `optional`s

Section: 20.6.3.5 [optional.swap], 20.6.9 [optional.specalg] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-19 Last modified: 2017-07-30

Priority: 0

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Discussion:

`optional` didn't benefit from the wording modifications by P0185 "Adding [`nothrow_`]`swappable` traits"; as such, it suffers from LWG 2456, and does not play nice with swappable traits.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4606.

1. Modify [optional.object.swap] as indicated:

   >     void swap(optional<T>& rhs) noexcept(see below);

   > > […]

   > > -4- Remarks: The expression inside `noexcept` is equivalent to:

   > > >     is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))

2. Modify 20.6.9 [optional.specalg] as indicated:

   >     template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));

   > > -1- Effects: Calls `x.swap(y)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true` and `is_swappable_v<T>` is `true`.