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.

Beginne mit der verlinkten TypeScript Playground-Reproduktion und vergleiche den inferierten Typ für die Tupel-Destrukturierung mit dem Ergebnis des indexierten Zugriffs. Sieh dir das zugehörige Issue #55661 als Kontext an. Die Aufgabe ist abgeschlossen, wenn der destrukturierte Wert in diesem Fall einer Union aus Array und Tupel als potenziell undefined gemeldet wird, ohne das korrekte Verhalten des indexierten Zugriffs zu ändern.