Type Guards do not narrow `this` parameters with union types.

[louisditzel]

TypeScript Version: 2.2.1

Code

interface A {
    a: number;
}

interface B {
    b: number;
}

function isA(arg: any): arg is A {
    return arg.a !== undefined;
}

function test(arg: A | B) {
    if (isA(arg)) {
        console.log(arg.a);
    } else {
        console.log(arg.b);
    }
}

function testThis(this: A | B) {
    if (isA(this)) {
        console.log(this.a);
    } else {
        console.log(this.b);
    }
}

Expected behavior:
arg is of type A or B in the function test, after passing the type guard.
this is of type A or B in the function testThis, after passing the type guard.

Actual behavior:
arg is the correct type.
this remains of type A | B, so typescript reports an error on both calls to this.a and this.b.

This can be seen in the playground here.