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Narrowed union types loosened within closures #19683

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@myitcv

Description

@myitcv

This could be a dup of #14748; they seem related

TypeScript Version: 2.7.0-dev.20171102

Code

See in playground

class A { 
    name: string;
}
class B { 
    age: number;
}

let x: A | B;

if (x instanceof A) {
    console.log(x.name); // ok: x has type A

    setTimeout(() => {
        console.log(x.name); // ** COMPILE ERROR: x has type A | B here
    });
} else {
    console.log(x.age); // ok: x has type B here

    setTimeout(() => {
        console.log(x.age); // ** COMPILE ERROR: x has type A | B here
    });
}

Expected behavior:

This should compile.

Actual behavior:

It does not compile

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