signDisplay with formatRange

[sffc]

What should happen here?

const nf = new Intl.NumberFormat("en", {
  signDisplay: "always"
});

console.log(nf.formatRange(50, 70));
console.log(nf.formatRange(50, 50));

Options:

Description	50 - 70	50 - 50
Always obey signDisplay	+50–70	+50
Obey on identity fallback	50–70	+50
Ignore	50–70	50

CLDR does not clearly specify this case.

If we picked the second option (obey only on identity fallback), it would work nicely with #6 and #10, to let you toggle back and forth between the two main options for identityFallback.

[sffc]

2020-10-08 discussion: https://github.com/tc39/ecma402/blob/master/meetings/notes-2020-10-08.md#signdisplay-with-formatrange-13

[sffc]

We should do option 1. This is consistent with the discussion on 2020-10-08.