Wrong type narrowing when types are compatibles

[akarzazi]

TypeScript Version: 2.0.3

Code

class Result1 {
    readonly value1: number;

    constructor(value1: number) {
        this.value1 = value1;
    }
}

class Result2 {
    readonly value1: number;
    readonly value2: number;

    constructor(value1: number, value2: number) {
        this.value1 = value1;
        this.value2 = value2;
    }
}

function someFunction(result: Result1 | Result2) {
    if (result instanceof Result1) {
        // result is still Result1 | Result2
        console.log("1");
    }
    else {
        // result is never ! this is wrong !
        console.log("2");
    }
}

// this will output 2 !
someFunction(new Result2(9,9))

Expected behavior:
instance of type guard should not care about type compatibility
Actual behavior:
Wrong assertion by the compiler.

[yortus]

Essentially a duplicate of #10934. See also design notes about instanceof in #8503.

[akarzazi]

Indeed, this is a duplicate :(
From #7271, the issue will be fixed in 2.1.

[yortus]

@akarzazi note that the fix for #7271 only affects narrowing structurally identical types, but your issue here (like #10934) is with narrowing structural subtype/supertypes. This hasn't been solved yet and is on the roadmap for future investigation at an unspecified time.

[akarzazi]

@yortus I thought the fix will cover type compatibly as well.
Since mhegazy said : As noted in #8503, classes should be treated definitely for instanceof checks.

[mikelehen]

FYI- This still reproduces with TypeScript 2.3.2

[nodefish]

Really simplistic example of what's wrong with instanceof:

const model = this.opts.resources.model.store.get(id);
if (!model || model instanceof Placeholder)
    return null;
const data: any = model.get();

TSC gives the following error:
Property 'get' does not exist on type 'never'.

A workaround was proposed in #13325 but I can't see an equivalent in imperative code sections.

[RyanCavanaugh]

Fixed in #19671