Trying to flatten an array with unknown depth causes a type error

Bug Report

Trying to flatten an array with unknown depth causes a type error.

🔎 Search Terms

• flat
• Infinity

🕗 Version & Regression Information

• Errors in version 4.2.4
• This changed between versions 3.6.2 and 3.7.5 (but probably just because the type definitions aren't supported prior)

The PR https://github.com/microsoft/TypeScript/pull/32131 provides a definition for FlatArray that has this issue.

⏯ Playground Link

Playground link with relevant code

💻 Code

// first define a nested list with arbitrary depth
type Nested<T> = (T | Nested<T>)[];

const arr: Nested<number> = [1,[2,[3]]];

// locally, this bit fails for me (version 4.2.4)
//
// with an error '"recur"' is referenced directly or indirectly in its own type annotation.
const res: number[] = arr.flat(Infinity);

// you can do a quick workaround as follows:
const res2: number[] = (arr as any[]).flat(Infinity);

// but for some reason on the TypeScript playground this doesn't fail, but you can emulate it
// by using the following definition that is equivalent to the one provided in the standard library:
// https://github.com/microsoft/TypeScript/blob/d8e9f6951919a347bb155b938a5006f9efabb778/lib/lib.es2019.array.d.ts#L21
type FlatArray2<Arr, Depth extends number> = {
    "done": Arr,
    "recur": Arr extends ReadonlyArray<infer InnerArr>
        ? FlatArray2<InnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]>
        : Arr
}[Depth extends -1 ? "done" : "recur"];

// causes a type error: '"recur"' is referenced directly or indirectly in its own type annotation.
type TestFlat<T> = FlatArray2<Nested<T>, typeof Infinity>

🙁 Actual behavior

The code errors with '"recur"' is referenced directly or indirectly in its own type annotation.

🙂 Expected behavior

The code to pass typechecking.

Riproduci l’errore con l’esempio fornito di array annidato ed esamina la definizione di FlatArray in lib/lib.es2019.array.d.ts, insieme a PR 32131. Determina in che modo la tipizzazione di flat per una profondità sconosciuta dovrebbe evitare l’errore di tipo ricorsivo, quindi verifica che arr.flat(Infinity) superi il controllo dei tipi senza indebolire il tipo di risultato previsto.