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. Kitson Kelly (@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.