instanceof type guard does not narrow type of `this`

[sccolbert]

Here's an example which demonstrates (tested on 1.6.2 and current playground):

class Foo {
    a: number;

    foo(): string {
        if (this instanceof Bar) {
            return this.b;  // not okay - `this` typed as Foo
        }
        return '';
    }

    bar(): string {
        let that = this;
        if (that instanceof Bar) {
            return that.b;  // okay - `that` typed as Bar
        }
        return ''
    }
}


class Bar extends Foo {
    b: string;
}

[kitsonk]

I think this is all related to polymorphic this #4910 which isn't in 1.6.2 (and playground is 1.6.2). Before that, you couldn't change the this type.

[ahejlsberg]

Type guards currently only affect parameters and local variables, but I agree it seems reasonable to also have them apply to this. @kitsonk No, not related to the polymorphic this feature, the restriction is still there in current builds.

[mhegazy]

This should be working as expected in master.