Proposed resolution:

This wording is relative to N4885.

1. Modify [format.context] as indicated:

   iterator out();
   

   -7- Returns: out_.Effects: Equivalent to: return std::move(out_);

   void advance_to(iterator it);
   

   -8- Effects: Equivalent to: out_ = std::move(it);

[ 2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-06-23; Reflector poll ]

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

LWG 3539 fixed copies of potentially-move-only iterators in the format_to and vformat_to overloads, but missed the fact that member functions of basic_format_context are specified to copy iterators as well. In particular, [format.context] states:

iterator out();

-7- Returns: out_.

void advance_to(iterator it);

-8- Effects: Equivalent to: out_ = it;

both of which appear to require copyability.