[lib.d.ts] Incorrect typing for CanvasPattern

[CookieShade]

TypeScript Version: 1.8.34

Code

let canvas = document.createElement("canvas");
let context = canvas.getContext("2d");

let pattern: CanvasPattern = []; // !!!
context.fillStyle = pattern;
context.strokeStyle = 0; // !!!

Expected behavior:
Fails to compile due to [] and 0 not being valid CanvasPatterns.

Actual behavior:
Successfully compiles, probably due to CanvasPattern being declared as an empty interface, which is obviously implemented by all objects. At runtime, the style is solid black.

// lib.d.ts
interface CanvasPattern {
}
//...
fillStyle: string | CanvasGradient | CanvasPattern;
//...
strokeStyle: string | CanvasGradient | CanvasPattern;

[DanielRosenwasser]

Looks like it should have a tag or something.

For reference: https://developer.mozilla.org/en-US/docs/Web/API/CanvasPattern

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[CookieShade]

I'd make a PR, but I lack the Typescript experience and know-how to semantically fix the issue.

[mhegazy]

potentially put a readonly _tag: never field on the interface, that would make it unassingable from other types.