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4399. enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t

Section: 28.5.9 [format.tuple] Status: New Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-10-04

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<Ts> && ...);

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++.

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<Ts>> && ...);
   }