Proposed resolution:

This wording is relative to N4917.

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

   constexpr size_t next_arg_id();
   

   -7- Effects: If indexing_ != manual is true, equivalent to:

   if (indexing_ == unknown)
     indexing_ = automatic;
   return next_arg_id_++;
   

   -8- Throws: format_error if indexing_ == manual is true which indicates mixing of automatic and manual argument indexing.

   -?- Remarks: Let cur-arg-id be the value of next_arg_id_ prior to this call. Call expressions where cur-arg-id >= num_args_ is true are not core constant expressions ([expr.const]).

   constexpr size_t check_arg_id(size_t id);
   

   -9- Effects: If indexing_ != automatic is true, equivalent to:

   if (indexing_ == unknown)
     indexing_ = manual;
   

   -10- Throws: format_error if indexing_ == automatic is true which indicates mixing of automatic and manual argument indexing.

   -11- Remarks: Call expressions where id >= num_args_ is true are not core constant expressions ([expr.const]).

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

[ Kona 2022-11-11; move to Ready ]

[ 2022-11-11; Tomasz provide improved wording; Move to Open ]

Clarify that the value of next_arg_id_ is used, and add missing "is true."

The definition of check_arg_id in [format.parse.ctx] includes a (compile-time) argument id check in the Remarks element:

constexpr void check_arg_id(size_t id);

[…]

Remarks: Call expressions where id >= num_args_ are not core constant expressions ([expr.const]).

However, a similar check is missing from next_arg_id which means that there is no argument id validation in user-defined format specification parsing code that invokes this function (e.g. when parsing automatically indexed dynamic width).

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

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

   constexpr size_t next_arg_id();
   

   -7- Effects: If indexing_ != manual, equivalent to:

   if (indexing_ == unknown)
     indexing_ = automatic;
   return next_arg_id_++;
   

   -8- Throws: format_error if indexing_ == manual which indicates mixing of automatic and manual argument indexing.

   -?- Remarks: Call expressions where next_arg_id_ >= num_args_ are not core constant expressions ([expr.const]).