[Proposal] Add static properties for Locale.NumberingSystem

Author: glebfann

Proposals/0012-numbering-system-static-properties.md

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+# Feature name
+
+* Proposal: [SF-0012](0012-numbering-system-static-properties.md)
+* Authors: [Gleb Fandeev](https://github.com/glebfann)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Implementation: [apple/swift-foundation#1055](https://github.com/swiftlang/swift-foundation/pull/1055)
+* Review: ([pitch](https://forums.swift.org/t/pitch-add-static-properties-for-locale-numberingsystem/76203))
+
+## Introduction
+
+This proposal adds static properties to `Locale.NumberingSystem` for all standard numbering systems defined in [Unicode CLDR](https://github.com/unicode-org/cldr/blob/latest/common/bcp47/number.xml), making it easier to work with different numbering systems in Swift.
+
+## Motivation
+
+Currently, to use a specific numbering system, developers need to create instances using string identifiers:
+```swift
+let arabic = Locale.NumberingSystem("arab")
+```
+This approach has several drawbacks:
+- Lack of Discoverability: Developers may not be aware of all available numbering systems or their corresponding identifiers.
+- Error-Prone: Manually typing string identifiers increases the risk of typos and mistakes.
+- Reduced Readability: String literals provide less context compared to well-named constants.
+- Inconsistency: Other Locale components like `Locale.LanguageCode`, `Locale.Region`, and `Locale.Script` already provide static properties for common identifiers, but `Locale.NumberingSystem` does not.
+
+By introducing predefined static properties for each numbering system, we can improve code safety, discoverability, readability, and maintain consistency across the Locale API.
+
+## Proposed solution
+
+Extend `Locale.NumberingSystem` to include static properties for each numbering system defined in the Unicode CLDR.
+
+Example usage:
+```swift
+let numberingSystem = Locale.NumberingSystem.arabicIndic
+```
+This allows developers to:
+
+- Use autocomplete features to discover available numbering systems.
+- Reduce typos and mistakes by avoiding manually typed strings: `let numberingSystem = Locale.NumberingSystem("arabic") // Incorrect identifier`
+- Improve code clarity with descriptive property names. For example, `Locale.NumberingSystem.simplifiedChinese` instead of `Locale.NumberingSystem("hans")`
+
+## Detailed design
+
+Add an extension to `Locale.NumberingSystem` containing static properties for each numbering system. The identifiers are sourced from the Unicode CLDR's numbering systems registry.
+
+```swift
+@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+extension Locale.NumberingSystem {
+    /// Adlam digits.
+    /// - Identifier: `"adlm"`.
+    @_alwaysEmitIntoClient
+    public static var adlam: Locale.NumberingSystem { Locale.NumberingSystem("adlm") }
+
+    /// Ahom digits.
+    /// - Identifier: `"ahom"`.
+    @_alwaysEmitIntoClient
+    public static var ahom: Locale.NumberingSystem { Locale.NumberingSystem("ahom") }
+
+    /// Arabic-Indic digits.
+    /// - Identifier: `"arab"`.
+    @_alwaysEmitIntoClient
+    public static var arabicIndic: Locale.NumberingSystem { Locale.NumberingSystem("arab") }
+
+    /// Extended Arabic-Indic digits.
+    /// - Identifier: `"arabext"`.
+    @_alwaysEmitIntoClient
+    public static var arabicIndicExtended: Locale.NumberingSystem { Locale.NumberingSystem("arabext") }
+
+    // ... all other numbering systems
+}
+```
+The full list can be viewed in the [implementation pull request](https://github.com/swiftlang/swift-foundation/pull/1055). Variable names are assigned based on the descriptions provided in the [Unicode CLDR](https://github.com/unicode-org/cldr/blob/latest/common/bcp47/number.xml).
+
+## Source compatibility
+
+These changes are additive only and are not expected to have an impact on source compatibility.
+
+## Implications on adoption
+
+This feature can be freely adopted and un-adopted in source code with no deployment constraints and without affecting source compatibility.
+
+## Acknowledgments
+
+Thanks to [Alobaili](https://forums.swift.org/u/alobaili/summary) for highlighting this issue in their [comment](https://forums.swift.org/t/fou-locale-components-language-and-language-components/54084/17) on the Swift forums, which inspired this proposal.