Proposed resolution:

This wording is relative to N4842.

1. Edit [format.syn], header <format> synopsis, as indicated:

   namespace std {
   
     […]
   
     template<class Out>
       Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
     template<class Out>
       Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
     template<class Out>
       Out vformat_to(Out out, const locale& loc, string_view fmt,
                      format_args_t<type_identity_t<Out>, char> args);
     template<class Out>
       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
                      format_args_t<type_identity_t<Out>, wchar_t> args);
   
     […]
   }
   

2. Edit [format.functions] p8 through p10 as indicated:

   template<class Out, class... Args>
     Out format_to(Out out, string_view fmt, const Args&... args);
   template<class Out, class... Args>
     Out format_to(Out out, wstring_view fmt, const Args&... args);
   

   -8- Effects: Equivalent to:

   using context = basic_format_context<Out, decltype(fmt)::value_type>;
   return vformat_to(out, fmt, {make_format_args<context>(args...)});
   

   template<class Out, class... Args>
     Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
   template<class Out, class... Args>
     Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);
   

   -9- Effects: Equivalent to:

   using context = basic_format_context<Out, decltype(fmt)::value_type>;
   return vformat_to(out, loc, fmt, {make_format_args<context>(args...)});
   

     template<class Out>
       Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
     template<class Out>
       Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
     template<class Out>
       Out vformat_to(Out out, const locale& loc, string_view fmt,
                      format_args_t<type_identity_t<Out>, char> args);
     template<class Out>
       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
                      format_args_t<type_identity_t<Out>, wchar_t> args);
   

   -10- Let charT be decltype(fmt)::value_type.

   […]

[ 2020-02-01 Status set to Tentatively Ready after six positive votes on the reflector. ]

vformat_to currently deduces Out from its first and last arguments. This requires its last argument's type to be a specialization of basic_format_args, which notably prevents the use of format-arg-store arguments directly. This is unnecessary: we should only deduce from the first argument.