4159. Uses-allocator construction mechanisms should be opted out for node handles

Section: 23.2.5 [container.node], 20.2.8.1 [allocator.uses.trait] Status: New Submitter: Jiang An Opened: 2024-09-21 Last modified: 2024-10-09

Priority: 3

View all issues with New status.

Discussion:

Currently, node handle types (23.2.5 [container.node]) have the member allocator_type type alias, which makes std::uses_allocator report true for them. However, they don't have allocator-extended constructors, and it's unclear to me whether it's better to make them have such constructors.

The status quo renders the following seemingly reasonable program ill-formed.

#include <set>
#include <map>

int main() {
  std::pmr::set<int> s{1};
  std::pmr::map<int, decltype(s)::node_type> m;
  m.emplace(1, s.extract(1));
}

Perhaps we should consistently treat node handles not uses-allocator-constructible.

[2024-10-09; Reflector poll]

Set priority to 3 after reflector poll.

"Strike the carve outs for program-defined specializations." If it's broken for uses_allocator<NH, A> to be true when NH is not a program-defined type, then we don't want to allow users to make it true for a program-defined A, that would still be broken.

Previous resolution [SUPERSEDED]:

This wording is relative to N4988.

1. Modify 23.2.5.1 [container.node.overview] as indicated:

   […]

   -4- If a user-defined specialization of pair exists for pair<const Key, T> or pair<Key, T>, where Key is the container's key_type and T is the container's mapped_type, the behavior of operations involving node handles is undefined.

   -?- For each node handle type NH and any type A, if uses_allocator<NH, A> is not a program-defined specialization, it meets the Cpp17BinaryTypeTrait requirements (21.3.2 [meta.rqmts]) and its base characteristic is false_type.

[2024-10-09; Adjust wording as requested in reflector poll]

Proposed resolution:

This wording is relative to N4988.

1. Modify 23.2.5.1 [container.node.overview] as indicated:

   […]

   -4- If a user-defined specialization of pair exists for pair<const Key, T> or pair<Key, T>, where Key is the container's key_type and T is the container's mapped_type, the behavior of operations involving node handles is undefined.

   -?- For each node handle type NH and any type A, uses_allocator<NH, A> meets the Cpp17BinaryTypeTrait requirements (21.3.2 [meta.rqmts]) and its base characteristic is false_type.