Wrong type narrowing when types are compatibles

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.

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

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

Adel KARZAZI (@akarzazi)

Adel KARZAZI (@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.

Troy Gerwien (@yortus)

Troy Gerwien (@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.

FYI- This still reproduces with TypeScript 2.3.2

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.

Fixed in #19671

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Wrong type narrowing when types are compatibles #11664

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Wrong type narrowing when types are compatibles #11664

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yortus commented on Oct 16, 2016

akarzazi commented on Oct 16, 2016

yortus commented on Oct 16, 2016

akarzazi commented on Oct 16, 2016

mikelehen commented on May 4, 2017

nodefish commented on Jun 21, 2017

RyanCavanaugh commented on Nov 17, 2017

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