I changed part of the ToRawPrecision algorithm in ES2019 as part of Unified NumberFormat. However, in my work on NFv3, I realized that one change I made is unsound. This is in a crucial function, ToRawPrecision, which is used for all significant digits rounding.

Problem explanation: In ToRawPrecision, the e value is supposed to represent the power of 10 of the resolved number after rounding. However, my change in ES2020 computed e based on the input value before rounding.

Example values of x, p, n, and e, with xFinal for reference:

Formula: x ≈ n × 10^e–p+1

Practically, the ES2020 algorithm works in all cases except those where the number rounds up to the next magnitude. A spec-compliant ES2020 implementation would need to return "9.99" instead of "10.0" in the fifth row.

Please check my logic!

All browsers still implement it correctly, but I would like to restore the old algorithm as soon as possible. I consider this an editorial change because it is restoring correct behavior after a mess-up in ES2020. (2021-05-06: This is still normative.)

Can we sneak this into ES2021? (2021-05-06: No, don't push this into ES 2021.)