better autocompletion for dicriminated union as tuples

Search Terms

autocompletion, tuple, tagged union

Suggestion

Use Cases

https://www.typescriptlang.org/docs/handbook/advanced-types.html#discriminated-unions
Discriminated unions of tuples is just the same in semantic with discriminated unions with object introduced in the official document of typescript.
And the type check for it is working correctly for now.
But the autocompletion for tuples isn't as powerful as one for objects.
Sometimes discriminate unions as objects are verbose so it leads us to write some sort of constructors for each variant. But tuples can solve this problem, and it even looks more similar to haskell's data and rust's enum.

Examples

// discriminated union as tuples
type A = ["a", { a: number }] | ["b": { b: number }];
const a: A = ["a", { /* both a and b are suggested when you fill in here */ }];
const b: A = ["b", { a: 1 }]; // type check works correctly! typescript tell us it's invalid.

// discriminated union as objects
type C = {
    tag: "a",
    a: number
} | {
    tag: "b",
    b: number
};
const c: C = { tag: "b", /* only b is suggested! */ };

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.

Issue にリンクされている TypeScript ハンドブックの「discriminated-unions」セクションから始め、tuple の例と object の例を比較してください。tuple の discriminated unions が object 形式と同様にバリアント固有の自動補完を提供し、既存の型チェック動作を維持できれば、作業は完了です。