Proposed resolution:

This wording is relative to N4901.

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
     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);
     };
   }
   

   […]

   basic_format_arg<basic_format_context> arg(size_t id) const noexcept;
   

   -5- Returns: args_.get(id).

[ 2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP. ]

[ 2022-01-30; Reflector poll ]

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

basic_format_context::arg(size_t) simply returns args_.get(id) to get the elements of args_, where the type of args_ is basic_format_args<basic_format_context>. Since basic_format_args's get(size_t) is noexcept, this function can also be noexcept.