`Decimal.FormatStyle` does not always respect the locale's decimalSeparator

[StevenSorial]

Description
Decimal.FormatStyle does not always respect the locale's decimalSeparator, also, the same error occurs with FloatingPointFormatStyle

Steps to reproduce
1- Set the iPhone region to Germany.
2- Set the iPhone language to English (US).
3- Set the Number Format decimal separator to period instead of the default comma.

Expected behavior

    let number: Decimal = 1.5

    let newFormatter = Decimal.FormatStyle.number
    let newResult = number.formatted(newFormatter) // 1,5 ❌

    let oldFormatter = NumberFormatter()
    oldFormatter.numberStyle = .decimal

    let oldResult = oldFormatter.string(from: number as NSNumber)! // 1.5 ✅

Environment

• Swift version: 5.7
• Xcode version: 14.2
• Deployment target: iOS 16.3

[StevenSorial]

feedback: FB11980066

[AnthonyLatsis]

cc @parkera

[AnthonyLatsis]

@hamishknight Sorry to bother you with this. Could you transfer this too?

[hamishknight]

Sure

[itingliu]

We're blocked by ICU to be able to support this unfortunately

[StevenSorial]

Apologies for my ignorance, how that is the case if NumberFormatter is able to format it correctly?

[itingliu]

Apologies for my ignorance, how that is the case if NumberFormatter is able to format it correctly?

NumberFormatter uses a legacy ICU API, where we've moved onto a modern one for Decimal.FormatStyle

[justingiffard-impact]

Hiya, is there any progress on this? I feel like this is a pretty bug bug because we cannot format decimals into the users preferred format and worse still we cannot parse from a user typed string into a decimal?

[teameh]

Any progress on this? This also affects the Measurement formatting apis.

let oldFormatter = NumberFormatter()
oldFormatter.numberStyle = .decimal
oldFormatter.string(from: 1.5)! // 1.5 ✅

Locale.current.decimalSeparator // "." ✅

Measurement<UnitLength>.FormatStyle(width: .abbreviated)
    .format(Measurement(value: 1.5, unit: .meters)) // 1,5 ❌

Measurement<UnitLength>.FormatStyle(
    width: .abbreviated,
    locale: Locale.current,
    usage: .general,
    numberFormatStyle: .number.locale(Locale.current)
)
    .format(Measurement(value: 1.5, unit: .meters)) // 1,5 ❌

[itingliu]

Here are some issues tracking ICU support:
https://unicode-org.atlassian.net/browse/ICU-22733
https://unicode-org.atlassian.net/browse/ICU-22734

We could try workaround this by find and replace the decimal separator of the formatted string, or go back to the legacy ICU API that still supports this.

[nikitaame]

Running into this same issue with Decimal.FormatStyle.Currency.

What are the trade offs of going back to the legacy ICU API?

[kamvoick]

This also affects groupingSeparator which in some cases might be misleadingly treated as decimalSeparator.

Using TextField with format parameter causing same problem, when you can have different separator on keyboard than in the input. That way, adding next separator (dot) causing weird decimal number conversion.

Please, fix it or revert back to the legacy API.