Type Guard Issue with Array.prototype.fill, Array Constructor

Typegurd Issue with Array.prototype.fill, Array Constructor

TypeScript Version: 3.5.1

Search Terms:

• Array.prototype.fill
• fill
• ArrayConstructor
• array implicit any

Code

const foo: number[] = new Array(3).fill("foo"); // accepted

Actual behavior:
This code above is accepted because new Array(3) returns any[], and so fill("foo") returns any[].

I know giving type explicitly to Array like

const foo: number[] = new Array<number>(3).fill("foo"); // error

would work, but I believe the compiler should reject the first one. (This is TypeScript.)

Expected behavior:

Option A. Array.prototype.fill returns narrow type
replace declaration of Array.fill in lib.es2015.core.d.ts like fill<S extends T>(value: S, start?: number, end?: number): S[];, then

const foo: number[] = new Array(3).fill("foo");
// error: "Type 'string[]' is not assignable to type 'number[]'."
// because `fill` is resolved as `fill<string>(value: string, .....): string[]`

const bar = new Array(3).fill(0); // `bar` is resolved as `number[]`
bar.fill("bar"); // error: "Type '"bar"' is not assignable to type 'number'."

const baz: (number|string)[] = new Array(3).fill(0); // accepted.
baz.fill("baz"); // accepted.

Option B. Array Constructor never return implicit any
The problem is that Array constructor returns any[] implicitly. The first code is accepted even with --strict or any other options. It means we always have to care about "Array constructor returns any[]".

Something like #26188 could solve this issue.

Playground Link:
https://www.typescriptlang.org/play/#src=const%20foo%3A%20number%5B%5D%20%3D%20new%20Array(3).fill(%22foo%22)%3B

Related Issues:
#29604

Inizia riproducendo l’esempio nel TypeScript Playground collegato, quindi leggi le dichiarazioni di Array e ArrayConstructor in lib.es2015.core.d.ts. Esamina le issue correlate #29604 e #26188 e determina quale comportamento proposto è previsto. Il lavoro è concluso quando il comportamento accettato è specificato e coperto dai test TypeScript pertinenti.