Proposed resolution:

This wording is relative to N4917.

1. Modify [format.formattable] as indicated:

   -1- Let fmt-iter-for<charT> be an unspecified type that models output_iterator<const charT&> ([iterator.concept.output]).

   
     template<class T, class Context,
              class Formatter = typename Context::template formatter_type<remove_const_t<T>>>
       concept formattable-with =         // exposition only
         semiregular<Formatter> &&
           requires (Formatter& f, const Formatter& cf, T&& t, Context fc,
                    basic_format_parse_context<typename Context::char_type> pc)
          {
            { f.parse(pc) } -> same_as<typename decltype(pc)::iterator>;
            { cf.format(t, fc) } -> same_as<typename Context::iterator>;
          };
   
   template<class T, class charT>
     concept formattable =
       semiregular<formatter<remove_cvref_t<T>, charT>> &&
       requires(formatter<remove_cvref_t<T>, charT> f,
                const formatter<remove_cvref_t<T>, charT> cf,
                T t,
                basic_format_context<fmt-iter-for<charT>, charT> fc,
                basic_format_parse_context<charT> pc) {
           { f.parse(pc) } -> same_as<basic_format_parse_context<charT>::iterator>;
           { cf.format(t, fc) } -> same_as<fmt-iter-for<charT>>;
       };
       formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>>>;
   

2. Modify [format.arg] as indicated:

   namespace std {
     template<class Context>
     class basic_format_arg {
     public:
       class handle;
   
     private:
       using char_type = typename Context::char_type;                              // exposition only
   
       variant<monostate, bool, char_type,
               int, unsigned int, long long int, unsigned long long int,
               float, double, long double,
               const char_type*, basic_string_view<char_type>,
               const void*, handle> value;                                         // exposition only
   
       template<class T> explicit basic_format_arg(T&& v) noexcept;                // exposition only
       explicit basic_format_arg(float n) noexcept;                                // exposition only
       explicit basic_format_arg(double n) noexcept;                               // exposition only
       explicit basic_format_arg(long double n) noexcept;                          // exposition only
       explicit basic_format_arg(const char_type* s);                              // exposition only
   
       template<class traits>
         explicit basic_format_arg(
           basic_string_view<char_type, traits> s) noexcept;                       // exposition only
   
       template<class traits, class Allocator>
         explicit basic_format_arg(
           const basic_string<char_type, traits, Allocator>& s) noexcept;          // exposition only
   
       explicit basic_format_arg(nullptr_t) noexcept;                              // exposition only
   
       template<class T>
         explicit basic_format_arg(T* p) noexcept;                                 // exposition only
   

   …

   template<class T> explicit basic_format_arg(T&& v) noexcept;
   

   -4- Constraints: T satisfies formattable-with<Context>. The template specialization

   
   typename Context::template formatter_type<remove_cvref_t<T>>
   

   meets the BasicFormatter requirements ([formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

   
   typename Context::template formatter_type<remove_cvref_t<T>>()
     .format(declval<T&>(), declval<Context&>())
   

   shall be well-formed when treated as an unevaluated operand ([expr.context]).

   -?- Preconditions: If decay_t<T> is char_type* or const char_type*, static_cast<const char_type*>(v) points to a NTCTS ([defns.ntcts]).

   -5- Effects: Let TD be remove_const_t<T>.

   1. (5.1) — If if TD is bool or char_type, initializes value with v;

   2. (5.2) — otherwise, if TD is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);

   3. (5.3) — otherwise, if TD is a signed integer type ([basic.fundamental]) and sizeof(TD) <= sizeof(int), initializes value with static_cast<int>(v);

   4. (5.4) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);

   5. (5.5) — otherwise, if TD is a signed integer type and sizeof(TD) <= sizeof(long long int), initializes value with static_cast<long long int>(v);

   6. (5.6) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);

   7. (5.?) — otherwise, if TD is a standard floating-point type, initializes value with v;

   8. (5.?) — otherwise, if TD is a specialization of basic_string_view or basic_string and TD::value_type is char_type, initializes value with basic_string_view<char_type>(v.data(), v.size());

   9. (5.?) — otherwise, if decay_t<TD> is char_type* or const char_type*, initializes value with static_cast<const char_type*>(v);

   10. (5.?) — otherwise, if is_void_v<remove_pointer_t<TD>> is true or is_null_pointer_v<TD> is true, initializes value with static_cast<const void*>(v);

   11. (5.7) — otherwise, initializes value with handle(v).

   -?- [Note: Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]

   
   explicit basic_format_arg(float n) noexcept;
   explicit basic_format_arg(double n) noexcept;
   explicit basic_format_arg(long double n) noexcept;
   

   -6- Effects: Initializes value with n.

   
   explicit basic_format_arg(const char_type* s) noexcept;
   

   -7- Preconditions: s points to a NTCTS ([defns.ntcts]).

   -8- Effects: Initializes value with s.

   
   template<class traits>>
     explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
   

   -9- Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

   
   template<class traits, class Allocator>>
     explicit basic_format_arg(
       const basic_string<char_type, traits, Allocator>& s) noexcept;
   

   -10- Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

   
   explicit basic_format_arg(nullptr_t) noexcept;
   

   -11- Effects: Initializes value with static_cast<const void*>(nullptr).

   
   template<class T> explicit basic_format_arg(T* p) noexcept;
   

   -12- Constraints: is_void_v<T> is true.

   -13- Effects: Initializes value with p.

   -14- [Note: Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]

3. Modify [format.arg] p17 and p18 as indicated:

   template<class T> explicit handle(T&& v) noexcept;
   

   -17- Let

   1. — (17.1) TD be remove_cvrefconst_t<T>,

   2. — (17.2) const-formattable be true if typename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>()) is well-formed, otherwise false,

   3. — (17.3) TQ be const TD if const-formattable is true const TD satisfies formattable-with<Context> and TD otherwise.

   -18- Mandates: const-formattable || !is_const_v<remove_reference_t<T>> is true. TQ satisfies formattable-with<Context>.

   -19- Effects: Initializes ptr_ with addressof(val) and format_ with

   [](basic_format_parse_context<char_type>& parse_ctx,
      Context& format_ctx, const void* ptr) {
     typename Context::template formatter_type<TD> f;
     parse_ctx.advance_to(f.parse(parse_ctx));
     format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
                                    format_ctx));
   }
   

4. Modify [format.arg.store] p2 as indicated:

   template<class Context = format_context, class... Args>
     format-arg-store<Context, Args...> make_format_args(Args&&... fmt_args);
   

   -2- Preconditions: The type typename Context::template formatter_type<remove_cvref_t<T_i>> meets the BasicFormatter requirements ([formatter.requirements]) for each T_i in Args.

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

[ Issaquah 2023-02-07; LWG ]

Edited proposed resolution to remove extra = in concept definition and capitialize start of (5.1). Move to Immediate for C++23

[ 2023-01-11; Jonathan adds the missing edits to the class synopsis ]

[ 2022-11-29; Casey expands the issue to also cover make_format_args ]

[ 2022-11-10; Jonathan provides improved wording ]

[ 2022-10-19; Jonathan provides improved wording ]

During reflector prioritization it was pointed out that forwarding references are unnecessary (arguments are always lvalues), and so using T& would be simpler.

In order to resolve the overload ambiguities discussed in 3718 replace all unary constructor overloads with a single constructor that works for any formattable type.

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

1. Modify [format.arg] as indicated:

   template<class T> explicit basic_format_arg(T&& v) noexcept;
   

   -4- Constraints: T satisfies formattable<char_type>. The template specialization

   
   typename Context::template formatter_type<remove_cvref_t<T>>
   

   meets the BasicFormatter requirements ([formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

   
   typename Context::template formatter_type<remove_cvref_t<T>>()
     .format(declval<T&>(), declval<Context&>())
   

   shall be well-formed when treated as an unevaluated operand ([expr.context]).

   -?- Preconditions: If decay_t<T> is char_type* or const char_type*, static_cast<const char_type*>(v) points to a NTCTS ([defns.ntcts]).

   -5- Effects: Let TD be remove_const_t<T>.

   1. (5.1) — if TD is bool or char_type, initializes value with v;

   2. (5.2) — otherwise, if TD is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);

   3. (5.3) — otherwise, if TD is a signed integer type ([basic.fundamental]) and sizeof(TD) <= sizeof(int), initializes value with static_cast<int>(v);

   4. (5.4) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);

   5. (5.5) — otherwise, if TD is a signed integer type and sizeof(TD) <= sizeof(long long int), initializes value with static_cast<long long int>(v);

   6. (5.6) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);

   7. (5.?) — otherwise, if TD is a standard floating-point type, initializes value with v;

   8. (5.?) — otherwise, if TD is a specialization of basic_string_view or basic_string and TD::value_type is char_type, initializes value with basic_string_view<char_type>(v.data(), v.size());

   9. (5.?) — otherwise, if decay_t<TD> is char_type* or const char_type*, initializes value with static_cast<const char_type*>(v);

   10. (5.?) — otherwise, if is_void_v<remove_pointer_t<TD>> is true or is_null_pointer_v<TD> is true, initializes value with static_cast<const void*>(v);

   11. (5.7) — otherwise, initializes value with handle(v).

   -?- [Note: Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]

   
   explicit basic_format_arg(float n) noexcept;
   explicit basic_format_arg(double n) noexcept;
   explicit basic_format_arg(long double n) noexcept;
   

   -6- Effects: Initializes value with n.

   
   explicit basic_format_arg(const char_type* s) noexcept;
   

   -7- Preconditions: s points to a NTCTS ([defns.ntcts]).

   -8- Effects: Initializes value with s.

   
   template<class traits>>
     explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
   

   -9- Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

   
   template<class traits, class Allocator>>
     explicit basic_format_arg(
       const basic_string<char_type, traits, Allocator>& s) noexcept;
   

   -10- Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

   
   explicit basic_format_arg(nullptr_t) noexcept;
   

   -11- Effects: Initializes value with static_cast<const void*>(nullptr).

   
   template<class T> explicit basic_format_arg(T* p) noexcept;
   

   -12- Constraints: is_void_v<T> is true.

   -13- Effects: Initializes value with p.

   -14- [Note: Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]

2. Modify [format.arg] p17 and p18 as indicated:

   template<class T> explicit handle(T&& v) noexcept;
   

   -17- Let

   1. — (17.1) TD be remove_cvrefconst_t<T>,

   2. — (17.2) const-formattable be true if typename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>()) is well-formed, otherwise false,

   3. — (17.3) TQ be const TD if const-formattable is true const TD satisfies formattable<char_type> and TD otherwise.

   -18- Mandates: const-formattable formattable<const TD, char_type> || !is_const_v<remove_reference_t<T> is true.

   -19- Effects: Initializes ptr_ with addressof(val) and format_ with

   [](basic_format_parse_context<char_type>& parse_ctx,
      Context& format_ctx, const void* ptr) {
     typename Context::template formatter_type<TD> f;
     parse_ctx.advance_to(f.parse(parse_ctx));
     format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
                                    format_ctx));
   }
   

[ 2022-01-29; Reflector poll ]

Set priority to 3 after reflector poll.

Two suggestions to just change it to be T& because we don't need forwarding references here, and only accepting lvalues prevents forming dangling references.

Previous resolution [SUPERSEDED]:

This wording is relative to N4901.

1. Modify [format.arg] as indicated:

   template<class T> explicit basic_format_arg(T&& v) noexcept;
   

   -?- Let TD be remove_cvref_t<T>.

   -4- Constraints: The template specialization

   typename Context::template formatter_type<remove_cvref_t<T>TD>
   

   meets the BasicFormatter requirements ([formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

   typename Context::template formatter_type<remove_cvref_t<T>TD>()
     .format(declval<T&>(), declval<Context&>())
   

   shall be well-formed when treated as an unevaluated operand ([expr.context]).

   -5- Effects:

   1. (5.1) — if TD is bool or char_type, initializes value with v;

   2. (5.2) — otherwise, if TD is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);

   3. (5.3) — otherwise, if TD is a signed integer type ([basic.fundamental]) and sizeof(TD) <= sizeof(int), initializes value with static_cast<int>(v);

   4. (5.4) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);

   5. (5.5) — otherwise, if TD is a signed integer type and sizeof(TD) <= sizeof(long long int), initializes value with static_cast<long long int>(v);

   6. (5.6) — otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);

   7. (5.7) — otherwise, initializes value with handle(v).

P2418R2 changed basic_format_arg's constructor to take a forwarding reference but didn't change [format.arg]/5 which inspects various properties of T. Now that the deduced type can be cvref-qualified, they need to be removed before the checks.