Issue 4399: enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t (original) (raw)

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4399. enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t

Section: 28.5.9 [format.tuple] Status: WP Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-11-11

Priority: Not Prioritized

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Discussion:

The enable_nonlocking_formatter_optimization variable template is specialized only for _cv_-unqualified types. However, the specialization for pair and tuple does not remove the references and _cv_-qualifiers from the elements:

template<class... Ts> constexpr bool enable_nonlocking_formatter_optimization<_pair-or-tuple_<Ts...>> = (enable_nonlocking_formatter_optimization && ...);

As consequence pair<const std::string, int> or pair<const std::string&, int&> (map and flat_map reference types) will not use unbuffered prints.

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

[2025-10-17; Reflector poll]

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

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 28.5.9 [format.tuple] as indicated:

   -1- For each of pair and tuple, the library provides the following formatter specialization where _pair-or-tuple_ is the name of the template:

   namespace std {
   […]

   template<class... Ts>
   constexpr bool enable_nonlocking_formatter_optimization<_pair-or-tuple_<Ts...>> =
   (enable_nonlocking_formatter_optimization<remove_cvref_t> && ...);

}