Proposed resolution:

This wording is relative to N4917.

1. Modify [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 {
       formatter<T, charT> underlying_;                                          // exposition only
       basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", ");      // exposition only
       basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("["); // exposition only
       basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>("]"); // exposition only
   
     public:
       constexpr void set_separator(basic_string_view<charT> sep) noexcept;
       constexpr void set_brackets(basic_string_view<charT> opening,
                                   basic_string_view<charT> closing) noexcept;
       constexpr formatter<T, charT>& underlying() noexcept { return underlying_; }
       constexpr const formatter<T, charT>& underlying() const noexcept { return underlying_; }
   
       […]
     };
   }
   

   […]

   constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   

   -7- Effects: Equivalent to: separator_ = sep;

   constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;
   

   -8- Effects: Equivalent to:

   opening-bracket_ = opening;
   closing-bracket_ = closing;
   

2. Modify [format.range.fmtdef] as indicated:

   namespace std {
     template<ranges::input_range R, class charT>
     struct range-default-formatter<range_format::sequence, R, charT> {    // exposition only
     private:
       using maybe-const-r = fmt-maybe-const<R, charT>;                    // exposition only
       range_formatter<remove_cvref_t<ranges::range_reference_t<maybe-const-r>>,
                       charT> underlying_;                                 // exposition only
   
     public:
       constexpr void set_separator(basic_string_view<charT> sep) noexcept;
       constexpr void set_brackets(basic_string_view<charT> opening,
                                   basic_string_view<charT> closing) noexcept;
     
       […]
     };
   }
   

   constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   

   -1- Effects: Equivalent to: underlying_.set_separator(sep);.

   constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;
   

   -2- Effects: Equivalent to: underlying_.set_brackets(opening, closing);.

3. Modify [format.tuple] as indicated:

   namespace std {
     template<class charT, formattable<charT>... Ts>
     struct formatter<pair-or-tuple<Ts...>, charT> {
     private:
       tuple<formatter<remove_cvref_t<Ts>, charT>...> underlying_;               // exposition only
       basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", ");      // exposition only
       basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("("); // exposition only
       basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>(")"); // exposition only
   
     public:
       constexpr void set_separator(basic_string_view<charT> sep) noexcept;
       constexpr void set_brackets(basic_string_view<charT> opening,
                                   basic_string_view<charT> closing) noexcept;
   
       […]
     };
   }
   

   […]

   constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   

   -5- Effects: Equivalent to: separator_ = sep;

   constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;
   

   -6- Effects: Equivalent to:

   opening-bracket_ = opening;
   closing-bracket_ = closing;
   

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP. ]

[ 2023-01-06; Reflector poll ]

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

The set_separator and set_brackets of range_formatter only invoke basic_string_view's assignment operator, which is noexcept, we should add noexcept specifications for them.

In addition, its underlying function returns a reference to the underlying formatter, which never throws, they should also be noexcept.

Similar rules apply to range-default-formatter and formatter's tuple specialization.