Narrowed union types loosened within closures

[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

See #9998 and #11498.
In this case I think the behavior is correct; x is mutable so it could be different by the time the timeout is called. You can fix this by using const instead of let.

[myitcv]

In my.situation (and indeed the repro) it is not possible for x to have been mutated, but I can see that in general case that's harder to work out definitively. Thanks for the const tip.