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4398. enable_nonlocking_formatter_optimization should be disabled for container adaptors

Section: 23.6.2 [queue.syn], 23.6.5 [stack.syn] Status: Tentatively Ready Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-10-17

Priority: 2

View all issues with Tentatively Ready status.

Discussion:

As the standard currently defines formatters for queue, prioriy_queue, and stack enable_nonlocking_formatter_optimization is specialized to true for these adaptors per 28.5.6.4 [format.formatter.spec] p3:

Unless specified otherwise, for each type T for which a formatter specialization is provided by the library, each of the headers provides the following specialization: 

template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

However, formatting an adaptor requires formatting of the underlying range in terms of ranges::ref_view, and we disable the nonlocking_optimizations for all ranges, including ranges::ref_view.

This problem does not occur for the flat_set, flat_map adaptors, which are also ranges, but unlike stack etc. they do not have a specialized formatter. They use the formatter specialization for ranges and we already disable the optimization for that formatter.

The proposed wording has recently been implemented in gcc's libstdc++.

[2025-10-14; Reflector poll]

Set priority to 2 after reflector poll.

This is a duplicate of LWG 4146(i), with a different proposed resolution.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

  1. Modify 23.6.2 [queue.syn], header <queue> synopsis, as indicated:

    […]
// 23.6.13 [container.adaptors.format], formatter specialization for queue
template<class charT, class T, formattable<charT> Container>
  struct formatter<queue<T, Container>, charT>;
  
template<class T, class Container>
  constexpr bool enable_nonlocking_formatter_optimization<queue<T, Container>> = false;

// 23.6.4 [priority.queue], class template priority_queue
template<class T, class Container = vector<T>,
         class Compare = less<typename Container::value_type>>
  class priority_queue;
[…]
// 23.6.13 [container.adaptors.format], formatter specialization for priority_queue
template<class charT, class T, formattable<charT> Container, class Compare>
  struct formatter<priority_queue<T, Container, Compare>, charT>;
  
template<class T, class Container, class Compare>
  constexpr bool enable_nonlocking_formatter_optimization<priority_queue<T, Container, Compare>> = false;
[…]

  2. Modify 23.6.5 [stack.syn], header <stack> synopsis, as indicated:

    […]

// 23.6.13 [container.adaptors.format], formatter specialization for stack
template<class charT, class T, formattable<charT> Container>
  struct formatter<stack<T, Container>, charT>;

template<class T, class Container>
  constexpr bool enable_nonlocking_formatter_optimization<stack<T, Container>> = false;

[…]