Inconsistent length checking for tuples

[FrenchDilettante]

TypeScript Version: 3.1.x (and below)

Search Terms: spread length check, "bug got 0 or more"

Code

let canvasCtx: CanvasRenderingContext2D;
// Expected 4 arguments, but got 0 or more.
canvasCtx.fillRect(...[0, 0, 100, 100]);

// does not throw any error
const [x, y, w, h] = [0, 0, 100, 100];
canvasCtx.fillRect(x, y, w, h);

Expected behavior:
Being able to call a function by spreading an array, where the size of the array matches the number of requested arguments.

Actual behavior:
Typescript considers the spread to have "0 or more" elements, when it has a fixed number

Playground Link: http://www.typescriptlang.org/play/index.html#src=let%20canvasCtx%3A%20CanvasRenderingContext2D%3B%0D%0A%2F%2F%20does%20not%20work%0D%0AcanvasCtx.fillRect(...%5B0%2C%200%2C%20100%2C%20100%5D)%3B%0D%0A%0D%0A%2F%2F%20works%0D%0Aconst%20%5Bx%2C%20y%2C%20w%2C%20h%5D%20%3D%20%5B0%2C%200%2C%20100%2C%20100%5D%3B%0D%0AcanvasCtx.fillRect(x%2C%20y%2C%20w%2C%20h)%3B%0D%0A

Related Issues: #4130 [closed]

[ahejlsberg]

We infer tuple types for array literals only when there is a contextual type to indicate it. For example, the following works:

let canvasCtx: CanvasRenderingContext2D;
let args: [number, number, number, number] = [0, 0, 100, 100];
canvasCtx.fillRect(...args);

In your example, since there is no indication you want a tuple type, the inferred type for the array literal is number[]. This in turn leads to the "got 0 or more" error.

Also see discussions in #16656 and #27179.

[FrenchDilettante]

Correct, thank you 👍