Proposed resolution:

This wording is relative to N4861.

1. Modify [format.args] as indicated:

   -1- An instance of basic_format_args provides access to formatting arguments. Implementations should optimize the representation of basic_format_args for a small number of formatting arguments. [Note: For example, by storing indices of type alternatives separately from values and packing the former. — end note]

   basic_format_args() noexcept;
   

   -2- Effects: Initializes size_ with 0.

   […]

   basic_format_arg<Context> get(size_t i) const noexcept;
   

   -4- Returns: i < size_ ? data_[i] : basic_format_arg<Context>().

   [Note: Implementations are encouraged to optimize the representation of basic_format_args for small number of formatting arguments by storing indices of type alternatives separately from values and packing the former. — end note]

[ 2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP. ]

[ 2020-08-09; Reflector prioritization ]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

The note in the final paragraph of [format.args] gives encouragement to implementations, which is not allowed in a note.

It needs to be normative text, possibly using "should", or if left as a note could be phrased as "Implementations can optimize the representation […]".