Proposed resolution:

This wording is relative to N4885.

1. Modify [formatter.requirements], Table [tab:formatter], as indicated:

   Table 67: Formatter requirements [tab:formatter]

   Expression	Return type	Requirement
   …
   f.format(t, fc)	FC::iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out() and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).
   …

[ 2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-05-24; Reflector poll ]

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

[ 2021-05-20 Tim comments and updates wording ]

During reflector discussion Victor said that the formatter requirements should allow dependency on any of the format arguments in the context. The wording below reflects that.

[ 2020-07-12; Reflector prioritization ]

Set priority to 3 after reflector discussions.

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.

1. Modify [formatter.requirements], Table [tab:formatter], as indicated:

   Table 67: Formatter requirements [tab:formatter]

   Expression	Return type	Requirement
   …
   f.format(t, fc)	FC::iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out() and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), and the range [pc.begin(), pc.end()) from the last call to f.parse(pc), and fc.arg(n), where n is a size_t index value that has been validated with a call to pc.check_arg_id(n) in the last call to f.parse(pc).
   …

The requirements on the expression f.format(t, fc) in [tab:formatter] say

Formats t according to the specifiers stored in *this, writes the output to fc.out() and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).

Strictly speaking, this wording effectively forbids f.format(t, fc) from calling fc.arg(n), whose motivation is precisely to allow a formatter to rely on arguments different from t. According to this interpretation, there's no conforming way to implement the "{ arg-id }" form of the width and precision fields of standard format specifiers. Moreover, the formatter described in the example if paragraph [format.context]/8 would also be non-conforming.