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### 3677. Is a cv-qualified `pair` specially handled in uses-allocator construction?

Section: 20.10.7.2 [allocator.uses.construction] Status: New Submitter: Jiang An Opened: 2022-02-16 Last modified: 2022-03-04

Priority: 2

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Discussion:

It seems unclear whether cv-qualified `pair` specializations are considered as specializations of `pair` in 20.10.7.2 [allocator.uses.construction].

Currently MSVC STL only considered cv-unqualified pair types as such specializations, while libstdc++ accept both cv-unqualified and `const`-qualified pair types as such specializations. The resolution of LWG 3525 uses `remove_cv_t`, which possibly imply that the specialization of pair may be cv-qualified.

The difference can be observed via the following program:

>     #include <utility>
>     #include <memory>
>     #include <vector>
>     #include <cassert>

>     template<class T>
>     class payload_ator {

>       int payload{};

>     public:
>       payload_ator() = default;

>       constexpr explicit payload_ator(int n) noexcept : payload{n} {}

>       template<class U>
>       constexpr explicit payload_ator(payload_ator<U> a) noexcept : payload{a.payload} {}

>       friend bool operator==(payload_ator, payload_ator) = default;

>       template<class U>
>       friend constexpr bool operator==(payload_ator x, payload_ator<U> y) noexcept
>       {
>         return x.payload == y.payload;
>       }

>       using value_type = T;

>       constexpr T* allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }

>       constexpr void deallocate(T* p, std::size_t n) { return std::allocator<T>{}.deallocate(p, n); }

>       constexpr int get_payload() const noexcept { return payload; }
>     };

>     bool test()
>     {
>       constexpr int in_v = 42;
>       using my_pair_t = std::pair<int, std::vector<int, payload_ator<int>>>;
>       auto out_v = std::make_obj_using_allocator<const my_pair_t>(payload_ator<int>{in_v}).second.get_allocator().get_payload();
>       return in_v == out_v;
>     }

>     int main()
>     {
>       assert(test()); // passes only if a const-qualified pair specialization is considered as a pair specialization
>     }

[2022-03-04; Reflector poll]

Set priority to 2 after reflector poll.

Proposed resolution: