Feature: Composing spread operator on Union types and static_key function to enable generic capability

Related issues: #13542 #15759 (slightly different take and example on closed issues)

I originally posted on issue #15759 but decided that they were different enough to be looked at separately and that issue doesn't address a generic capability. Look back at https://github.com/Microsoft/TypeScript/issues/15759#issuecomment-314157616 for clunky workaround examples.

Proposal

1. Add spread operator ... to string literal union types acting as an string array or generator. Type T applying spread operator will result in type T[]. For example:

type Group = 'red' | 'blue' | 'green';
const groups = ...Group; // groups has type of Group[] with values ['red', 'green', 'blue']

2. Add static function static_key or keyof that transforms a static interface to a string literal union of all combined types. For example:

interface Groups<T, U, V> {
  red: T;
  green: U;
  blue: V;
}
type groups = static_key(Groups); // 'red' | 'blue' | 'green'

3. Composing both can result in powerful generic operations. I feel that this could bring a unique capability that string enums could not and may have been overlooked. It would be very powerful for a Record type or interface where all fields are the same type. Here is an example:

interface GroupDescription {
    name: string;
    description: string;
}

type Group = 'red' | 'blue' | 'green';
type Groups = { [K in Group]: GroupDescription };

function initGroups(groups: Groups) {
    for(const group of ...static_key(Groups)) { // group would be type Group or 'red' | 'blue' | 'green'
        processGroup(name); //Generically Process property
    }
}

function processGroup(group: Group) { }

I noticed I don't need ...static_key but instead directly use the spread operator on the union type ...Group. The point is that they compose well together and they add generic capability.

この issue にはファイル、テスト、エントリポイントが記載されていません。まず提案と関連する issue #13542 および #15759 を、リンクされている workaround のコメントも含めて読み、その後 union spreading と static_key の合意済みの設計を確立してください。完了とするには、スコープと実装基準について maintainer の承認が必要です。