Proposed resolution:

This wording is relative to N4849.

1. Modify the sign options Table [tab:format.sign] in [format.string.std] as indicated:

   Table 58: Meaning of sign options [tab:format.sign]

   Option	Meaning
   '+'	Indicates that a sign should be used for both non-negative and negative numbers.
   '-'	Indicates that a sign should be used only for negative numbers and negative zero only (this is the default behavior).
   space	Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

[ 2020-02 Status to Immediate on Thursday night in Prague. ]

[ 2020-02-13, Prague ]

Rebased and some wording finetuning by LWG.

[ 2019-08-17 Priority set to 2 based on reflector discussion ]

This wording is relative to N4830.

1. Modify the sign options Table [tab:format.sign] in [format.string.std] as indicated:

   Table 59: Meaning of sign options [tab:format.sign]

   Option	Meaning
   '+'	Indicates that a sign should be used for both non-negative and negative numbers.
   '-'	Indicates that a sign should be used only for negative numbers and negative zero (this is the default behavior).
   space	Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

What are these:

std::format("{}", -0.0);
std::format("{:+}", -0.0);
std::format("{:-}", -0.0);
std::format("{: }", -0.0);

with

"{:{}} {}"

A negative zero is not a negative number, so I think the answer for an implementation that supports signed zeroes is "0", "-0", "0", " 0". Is that the intent? (Note that this doesn't satisfy to_chars' round-trip guarantee.)

Or should the behavior actually be that "+" means "insert a leading + if to_chars' output does not start with -" and " " actually means "insert a leading space if to_chars' output does not start with -" (that is, the same as "%+f" and "% f") — so that the result of all four calls is "-0"?

Victor Zverovich:

The latter. std::format is specified in terms of to_chars and the intent is to have similar round trip guarantee here, so the result should be "-0", "-0", "-0", "-0". This has also been extensively discussed in LEWG and the outcome of that discussion was to print '-' for negative zeros.

So I think it should be clarified that '-' and space apply to negative numbers and negative zero.