Proposed resolution:

This wording is relative to N4971.

1. Modify [format.context] as indicated:

   namespace std {
     template<class Out, class charT>
     class basic_format_context {
       basic_format_args<basic_format_context> args_; // exposition only
       Out out_;                                      // exposition only
   
       basic_format_context(const basic_format_context&) = delete;
       basic_format_context& operator=(const basic_format_context&) = delete;
     public:
       using iterator = Out;
       using char_type = charT;
       template<class T> using formatter_type = formatter<T, charT>;
       
       basic_format_arg<basic_format_context> arg(size_t id) const noexcept;
       std::locale locale();
       
       iterator out();
       void advance_to(iterator it);
     };
   }
   

Per [format.context], it seems that std::basic_format_context has a default constructor that is effectively defaulted, which means that it is default constructible if and only if OutIt is default constructible. Currently only libstdc++ makes it conditionally default constructible, while libc++ and MSVC STL (together with fmtlib) make it never default constructible.

It seems that basic_format_context objects are supposed to be created by the implementation in some internal way, and user codes are only supposed to modify existing basic_format_context objects during formatting.