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3821. uses_allocator_construction_args should have overload for pair-like

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22

Priority: 2

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Discussion:

P2165R4 added a pair-like constructor to std::pair but didn't add a corresponding uses_allocator_construction_args overload. It was in P2165R3 but incorrectly removed during the small group review.

Without LWG 3525⁽ⁱ⁾, not having the overload would have caused emplacing a pair-like into a pmr::vector<pair> to be outright ill-formed.

With that issue's resolution, in cases where the constructor is not explicit we would create a temporary pair and then do uses-allocator construction using its pieces, and it still won't work when the constructor is explicit.

We should just do this properly.

[2022-11-09 Tim updates wording following LWG review]

During review of this issue LWG noticed that neither the constructor nor the new overload should accept subrange.

The remove_cv_t in the new paragraph is added for consistency with LWG 3677⁽ⁱ⁾.

[Kona 2022-11-12; Set priority to 2]

[2023-01-11; LWG telecon]

Replace P with U in p17 and set status to Tentatively Ready (poll result: 8/0/0).

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917 after the application of LWG 3677⁽ⁱ⁾.

1. Edit 22.3.2 [pairs.pair] as indicated:

   template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p);
   template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p);
   template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p);
   template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p);
   template<pair-like P> constexpr explicit(see below) pair(P&& p);
   

   -14- Let FWD(u) be static_cast<decltype(u)>(u).

   -15- Constraints:

   1. (15.?) — For the last overload, remove_cvref_t<P> is not a specialization of ranges::subrange,

   2. (15.1) — is_constructible_v<T1, decltype(get<0>(FWD(p)))> is true and

   3. (15.2) — is_constructible_v<T2, decltype(get<1>(FWD(p)))> is true.

   -16- Effects: Initializes first with get<0>(FWD(p)) and second with get<1>(FWD(p)).

2. Edit 20.2.2 [memory.syn], header <memory> synopsis, as indicated:

   namespace std {
     […]
     // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
     […]
     template<class T, class Alloc, class U, class V>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                       pair<U, V>& pr) noexcept;
   
     template<class T, class Alloc, class U, class V>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                       const pair<U, V>& pr) noexcept;
   
     template<class T, class Alloc, class U, class V>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                       pair<U, V>&& pr) noexcept;
   
     template<class T, class Alloc, class U, class V>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                       const pair<U, V>&& pr) noexcept;
     template<class T, class Alloc, pair-like P>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;
       
     template<class T, class Alloc, class U>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
     […]
   }
   

3. Add the following to 20.2.8.2 [allocator.uses.construction]:

     template<class T, class Alloc, pair-like P>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;
   

   -?- Constraints: remove_cv_t<T> is a specialization of pair and remove_cvref_t<P> is not a specialization of ranges::subrange.

   -?- Effects: Equivalent to:

   
   return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                               forward_as_tuple(get<0>(std::forward<P>(p))),
                                               forward_as_tuple(get<1>(std::forward<P>(p))));
   

4. Edit 20.2.8.2 [allocator.uses.construction] p17:

     template<class T, class Alloc, class U>
       constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
   

   -16- Let FUN be the function template:

     template<class A, class B>
     void FUN(const pair<A, B>&);
   

   -17- Constraints: remove_cv_t<T> is a specialization of pair, and either:

   1. (17.1) — remove_cvref_t<U> is a specialization of ranges::subrange, or

   2. (17.2) — U does not satisfy pair-like and the expression FUN(u) is not well-formed when considered as an unevaluated operand..