Potentially-`undefined` value not flagged when destructuring union of empty tuple and array

🔎 Search Terms

"tuple destructuring", "tuple narrowing"

🕗 Version & Regression Information

This bug appears to occur as far back as TS 3.3 up to the current nightly build, as tested in the Playground.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.2#code/C4TwDgpgBAggTnKBeWCCGIA8A7ArgWwCMI4A+KAHygG0BdSmgZ2DgEtsBzWgbgCheAJhADGAGzRxowgPbZmUCXABcqOH14y5wGmnopFfNADpg0gMot2HABQBKblAD0jqAFEAHpGHAIAlSThpOCNYb1w0URVsaSgAoJDAGXJ+TXlCZAUEagAGHl5CE3NLTjsHZzdPER8-WIR40OBwyJrA4KhAUHJ+XigoIA

💻 Code

type Arr = Array<number> | [] | [string];

declare const arr: Arr;

const [a] = arr;
a.toString(); // Expected: error. Actual: no error. ❌

const b = arr[0];
b.toString(); // Expected: error. Actual: error. ✅

.d.ts from Playground:

type Arr = Array<number> | [] | [string];
declare const arr: Arr;
declare const a: string | number | undefined;
declare const b: string | number | undefined;

🙁 Actual behavior

When destructuring a value whose type is an array, an empty tuple, and a non-empty tuple, the destructured value isn't flagged as possibly undefined which caused a runtime error. The value is correctly flagged as possibly undefined when accessing it by index.

🙂 Expected behavior

When destructuring a value whose type is an array, an empty tuple, and a non-empty tuple, the destructured value should be flagged as potentially being undefined.

Additional information about the issue

This seems related to #55661 except, in this case, the union contains a mix of tuples and an array.

リンクされた TypeScript Playground の再現例から始め、タプルの分割代入で推論される型と、インデックスアクセスの結果を比較してください。背景については、関連する issue #55661 を確認してください。この配列とタプルのユニオンの場合に、分割代入された値が undefined になる可能性があると報告され、正しいインデックスアクセスの動作が変更されていなければ完了です。