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### 2766. Swapping non-swappable types

Section: 20.4.3 [pairs.spec], 20.5.10 [tuple.special], 20.6.9 [optional.specalg], 20.7.10 [variant.specalg], 20.11.1.6 [unique.ptr.special], 22.3.7.4 [array.special], 22.6.4.5 [queue.special], 22.6.5.5 [priqueue.special], 22.6.6.6 [stack.special] Status: New Submitter: Agustín K-ballo Bergé Opened: 2016-08-15 Last modified: 2020-09-06

Priority: 3

View all other issues in [pairs.spec].

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Discussion:

Related: 2748 swappable traits for optionals, 2749 swappable traits for variants.

The adoption of P0185R1 "Adding [nothrow-]swappable traits" makes certain non-swappable types indirectly swappable. Consider a type defined as follows:

>     struct non_swappable {
>       friend void swap(non_swappable&, non_swappable&) = delete;
>     };

>     non_swappable ns1, ns2;
>     using std::swap;
>     swap(ns1, ns2); // ill-formed

>     static_assert(std::is_swappable_v<non_swappable> == false); // holds

Lvalues of type `non_swappable` are not swappable, as defined by 16.4.4.3 [swappable.requirements], overload resolution selects the deleted function. Consistently, `is_swappable_v<non_swappable>` yields false. It should be noted that since `non_swappable` is move constructible and move assignable, a qualified call to `std::swap` would be well-formed, even under P0185. Now consider the following snippet:

>     std::tuple<non_swappable> tns1, tns2;
>     using std::swap;
>     swap(tns1, tns2); // previously ill-formed, now well-formed

>     static_assert(std::is_swappable_v<std::tuple<non_swappable>> == false); // fires

Before P0185, this snippet would violate the implicit requirement of specialized swap for tuples that each tuple element be swappable. After P0185, this specialized swap overload for tuples would be SFINAEd away, resulting in overload resolution selecting the base swap overload, and performing the exchange via move construction and move assignment of tuples.

This issue affects all of `pair`, `tuple`, `unique_ptr`, `array`, `queue`, `priority_queue`, `stack`, and should eventually also apply to `optional` and `variant`. Previous resolution [SUPERSEDED]:

> This wording is relative to N4606, except when otherwise noted.

> 1. Modify 20.4.3 [pairs.spec] as indicated:

>    >     template<class T1, class T2> void swap(pair<T1, T2>& x, pair<T1, T2>& y)
>    >       noexcept(noexcept(x.swap(y)));

>    > > -7- Effects: As if by `x.swap(y)`.

>    > > -8- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<T1>` is `true` and `is_swappable_v<T2>` is `true`.

> 2. Modify 20.5.10 [tuple.special] as indicated:

>    >     template <class... Types>
>    >       void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

>    > > -1- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<`Ti`>` is `true` for all `i`, where `0 <= i` and `i < sizeof...(Types)`. The expression inside `noexcept` is equivalent to:

>    > > >     noexcept(x.swap(y))

>    > > -2- Effects: As if by `x.swap(y)`.

> 3. Modify 20.11.1.6 [unique.ptr.special] as indicated:

>    >     template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

>    > > -1- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<D>` is `true`.

>    > > -2- Effects: Calls `x.swap(y)`.

> 4. Modify 22.3.7.4 [array.special] as indicated:

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

>    > > -1- Remarks: This function shall not participate in overload resolution be defined as deleted unless `N == 0` or `is_swappable_v<T>` is `true`.

>    > > -2- Effects: As if by `x.swap(y)`.

>    > > […]

> 5. Modify 22.6.4.5 [queue.special] as indicated:

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

>    > > -1- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<Container>` is `true`.

>    > > -2- Effects: As if by `x.swap(y)`.

> 6. Modify 22.6.5.5 [priqueue.special] as indicated:

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

>    > > -1- `Remarks:` This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<Container>` is `true` and `is_swappable_v<Compare>` is `true`.

>    > > -2- Effects: As if by `x.swap(y)`.

> 7. Modify 22.6.6.6 [stack.special] as indicated:

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

>    > > -1- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_swappable_v<Container>` is `true`.

>    > > -2- Effects: As if by `x.swap(y)`.

> 8. Modify 20.6.9 [optional.specalg] as indicated:

>    > This change should be performed if and only if LWG 2748 is accepted and is against the wording of 2748:

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

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

>    > > -2- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_move_constructible_v<T>` is `true` and `is_swappable_v<T>` is `true`.

> 9. Modify 20.7.10 [variant.specalg] as indicated:

>    > This change should be performed if and only if LWG 2749 is accepted and is against the wording of 2749:

>    >     template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

>    > > -1- Effects: Equivalent to `v.swap(w)`.

>    > > -2- Remarks: This function shall not participate in overload resolution be defined as deleted unless `is_move_constructible_v<Ti> && is_swappable_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to `noexcept(v.swap(w))`.

[2019-04-17 Jonathan updates proposed resolution based on Ville's 2016-11-17 observation that the container adaptors always require swappable sequences anyway. The new proposed resolution is based on the latest WP, "de-shalled", and Remarks elements are repositioned after the Effects.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 20.4.3 [pairs.spec] as indicated:

   >     template<class T1, class T2>
   >       constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));

   > > -7- Effects: As if by `x.swap(y)`.

   > > -8- Remarks: This function shall not participate in overload resolution is defined as deleted unless `is_swappable_v<T1>` is `true` and `is_swappable_v<T2>` is `true`.

2. Modify 20.5.10 [tuple.special] as indicated:

   >     template <class... Types>
   >       constexpr void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

   > > -?- Effects: As if by `x.swap(y)`.

   > > -1- Remarks: This function shall not participate in overload resolution is defined as deleted unless `is_swappable_v<`Ti`>` is `true` for all `i`, where `0 <= i` and `i < sizeof...(Types)`. The expression inside `noexcept` is equivalent to:

   > > >     noexcept(x.swap(y))

   > > -2- Effects: As if by `x.swap(y)`.

3. 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)`.

   > > -2- Remarks: This function shall not participate in overload resolution is defined as deleted unless `is_move_constructible_v<T>` is `true` and `is_swappable_v<T>` is `true`.

4. Modify 20.7.10 [variant.specalg] as indicated:

   >     template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

   > > -1- Effects: Equivalent to `v.swap(w)`.

   > > -2- Remarks: This function shall not participate in overload resolution is defined as deleted unless `is_move_constructible_v<Ti> && is_swappable_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to `noexcept(v.swap(w))`.

5. Modify 20.11.1.6 [unique.ptr.special] as indicated:

   >     template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

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

   > > -1- Remarks: This function shall not participate in overload resolution is defined as deleted unless `is_swappable_v<D>` is `true`.

   > > -2- Effects: Calls `x.swap(y)`.

6. Modify 22.3.7.4 [array.special] as indicated:

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

   > > -1- Constraints: `N == 0` or `is_swappable_v<T>` is `true`.

   > > -2- Effects: As if by `x.swap(y)`.

   > > -3- Complexity: Linear in `N`.

   > > -?- Remarks: This function is defined as deleted unless `N == 0` or `is_swappable_v<T>` is `true`.