static toString with incorrect type breaks instanceof

[kieferrm]

See microsoft/vscode#16819

TypeScript Version: typescript@2.1.4-insiders.20161206

• VSCode Version: Code - Insiders 1.8.0-insider (0d65ced, 2016-12-07T07:05:09.124Z)
• OS Version: Darwin x64 15.6.0

Code

export abstract class Marker {
	static toString(markers: Marker[]): string {
		return '';
	}
}

export class Placeholder extends Marker {
	constructor(public name: string = '', public defaultValue: Marker[]) {
		super();
	}
}

function foo(marker: Marker) {
	if (marker instanceof Placeholder) {
		console.log(marker.name);
		console.log(marker.defaultValue);
	}
}

foo(null);

Expected behavior:
No erros are shown in the vscode editor

Actual behavior:

When the static toString is removed or renamed the problem goes away.

[sandersn]

1. The type of toString is incorrect. markers is required and should be optional, because toString might get called on the class (not its instance). You can make markers optional as a workaround.
2. We should probably relax the instance rhs requirement to just be a type that has a call or construct signature. It doesn't actually need to be assignable to Function.

-- OR --
2. Give an error when the instance or static side of a class incorrectly extends Object. But we don't do this today, and it would breaks tons of code. And people must not care too much, because I don't know of an existing bug for this.

[sandersn]

Fix is up at #12728

[about-code]

Give an error when the instance or static side of a class incorrectly extends Object. But we don't do this today, and it would breaks tons of code. And people must not care too much, because I don't know of an existing bug for this.

Maybe there is: #442

[sandersn]

As far as I can tell, #442 is essentially about using built-in names that are readonly, not incorrectly overriding a name that is meant to be overridden.

However, your PR to fix it looks undeservedly ignored. I'll take a look.

[about-code]

As far as I can tell, #442 is essentially about using built-in names that are readonly, not incorrectly overriding a name that is meant to be overridden.

@sandersn indeed, you're right. I think I got this mixed up. Thank you very much for your encouraging review.