Strict version of Extract for literal union types

Search Terms

extract, pick, union, literal, string

Suggestion

Currently we have Pick<T, K> to select a type-safe subset of a given type T using a union of keys K. Unfortunately this does not work for unions of literals and requires Extract which subjectively isn't strict enough:

type ShirtSizes = "xxxl" | "xxl" | "xl" | "l" | "m" | "s" | "xs" | "xxs";
type Subset = Extract<ShirtSizes, "l" | "m" | "s" | "this-is-allowed-as-well">;

One caveat is that Extract does not support code completion when writing the literal union for the keys and it allows keys not in the first type as well. The former is error-prone while the latter is not causing a different behavior since the illegal keys are ignored but it can't be strictly checked in the long run. Of course I understand there are use cases wherein you'd like to use Extract more liberally so the definition

type Extract<T, U> = T extends U ? T : never;

is reasonable enough. Nevertheless, I'd like to propose an additional strict variant as follows

// short-hand form
type ExtractStrict<T, U extends T> = Extract<T, U>;
// equivalent full form
type ExtractStrict<T, U extends T> = T extends U ? T : never;

Use Cases

type ShirtSizes = "xxxl" | "xxl" | "xl" | "l" | "m" | "s" | "xs" | "xxs";
// this allows for code completion
type Subset = ExtractStrict<ShirtSizes, "l" | "m" | "s">;
// this is causing an error
type Subset2 = ExtractStrict<ShirtSizes, "l" | "m" | "s" | "illegal-literal">;

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

まず、既存の Extract ユーティリティ型と issue の ShirtSizes の例を確認します。無効なリテラルの拒否や補完のサポートを含め、strict なバリアントが TypeScript の型システムの設計とテスト動作にどのように適合するかを判断します。提案する型が仕様化、実装され、コンパイラーテストでカバーされていれば完了です。