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4246. Redundant constraint in range_formatter::format

Section: 28.5.7.2 [format.range.formatter] Status: New Submitter: Hewill Kang Opened: 2025-04-18 Last modified: 2025-04-21

Priority: Not Prioritized

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Discussion:

Currently, the signature of range_formatter::format is as follows:

template<ranges::input_range R, class FormatContext>
  requires formattable<ranges::range_reference_t<R>, charT> &&
           same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
typename FormatContext::iterator
  format(R&& r, FormatContext& ctx) const;

which requires that the reference type of the range parameter must be formattable, and such type must be exactly T after removing the cvref-qualifiers.

However, satisfying the latter always implies satisfying the former, as the range_formatter class already requires that T must be formattable.

There is no need to perform a redundant check here.

Proposed resolution:

This wording is relative to N5008.

1. Modify 28.5.7.2 [format.range.formatter] as indicated:

   namespace std {
     template<class T, class charT = char>
       requires same_as<remove_cvref_t<T>, T> && formattable<T, charT>
     class range_formatter {
       […]
       template<ranges::input_range R, class FormatContext>
           requires formattable<ranges::range_reference_t<R>, charT> &&
                    same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
         typename FormatContext::iterator
           format(R&& r, FormatContext& ctx) const;
     };
   }
   

   […]

   template<ranges::input_range R, class FormatContext>
     requires formattable<ranges::range_reference_t<R>, charT> &&
              same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
   typename FormatContext::iterator
     format(R&& r, FormatContext& ctx) const;
   

   -11- Effects: Writes the following into ctx.out(), adjusted according to the range-format-spec: