[Bug libstdc++/90239] New: [C++20] scoped_allocator_adaptor should support nested pair

[kariya_mitsuru at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90239

            Bug ID: 90239
           Summary: [C++20] scoped_allocator_adaptor should support nested
                    pair
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com
  Target Milestone: ---

The current implementation of scoped_allocator_adaptor is not support nested
pair.

=========================== sample code ===========================
#include <iostream>
#include <vector>
#include <utility>
#include <scoped_allocator>
#include <new>

// stateful allocator
template <typename T>
class MyAlloc {
public:
    using value_type = T;
    T* allocate(std::size_t n) { return static_cast<T*>(::operator
new(sizeof(T) * n)); }
    void deallocate(T* p, std::size_t n) { ::operator
delete(static_cast<void*>(p), sizeof(T) * n); }
    MyAlloc(int state) noexcept : state(state) {}
    template <typename U>
    MyAlloc(const MyAlloc<U>& o) noexcept : state(o.state) {}
    int state;
};

template <typename T>
bool operator==(const MyAlloc<T>& lhs, const MyAlloc<T>& rhs) noexcept
{
    return lhs.state == rhs.state;
}

template <typename T>
bool operator!=(const MyAlloc<T>& lhs, const MyAlloc<T>& rhs) noexcept
{
    return lhs.state != rhs.state;
}

// type alias
template <typename T>
using SA = std::scoped_allocator_adaptor<MyAlloc<T>>;

template <typename T>
using VEC = std::vector<T, SA<T>>;

int main()
{
    VEC<std::pair<std::pair<VEC<int>, int>, int>> v(SA<int>(1));
    v.emplace_back(std::make_pair(VEC<int>{SA<int>(2)}, 1), 2);
    v.emplace_back(std::make_pair(VEC<int>{SA<int>(3)}, 3), 4);
    std::cout << v.get_allocator().state << ", "
              << v[0].first.first.get_allocator().state << ", "
              << v[1].first.first.get_allocator().state << '\n';
}
=========================== sample code ===========================
=========================== sample output ===========================
1, 2, 3
=========================== sample output ===========================
cf. https://wandbox.org/permlink/mcVJEFjpR2UeyW0O

I think that it should output "1, 1, 1" in c++2a mode.


P0591R4: Utility functions to implement uses-allocator construction
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0591r4.pdf

C++ 2020 Implementation Status
https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2020

commit for P0591R4
https://github.com/gcc-mirror/gcc/commit/6a9c77f14d18d819dc4b03e1ebc2da4e5f085627