Proposed resolution:

This wording is relative to N4988.

1. Modify [format.parse.ctx] as indicated:

   template<class... Ts>
     constexpr void check_dynamic_spec(size_t id) noexcept;
   

   -14- Mandates: `sizeof...(Ts)` ≥ 1. The types in `Ts...` are unique. Each type in `Ts...` is one of `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>, or `const void*`.

   -15- Remarks: A call to this function is a core constant expression only if:

   1. (15.1) — id < num_args_ is `true` and
   2. (15.2) — the type of the corresponding format argument (after conversion to basic_format_arg<Context>) is one of the types in `Ts...`.

[ 2024-09-18; Reflector poll ]

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

The Mandates: conditions for `format_parse_context::check_dynamic_spec` are:

-14- Mandates: The types in `Ts...` are unique. Each type in `Ts...` is one of `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>, or `const void*`.

There seems to be no reason to allow `Ts` to be an empty pack, that's not useful. There is no valid arg-id value that can be passed to it if the list of types is empty, since `arg(n)` will never be one of the types in an empty pack. So it's never a constant expression if the pack is empty.