Strange inference failure

[jods4]

TypeScript Version: 2.6.2

Code

let x: { nodes: DocumentFragment, _start: Comment };
let iter: Node | null, nodes: DocumentFragment;
({ nodes, _start: iter } = x!);
while (iter) {
    let next = iter.nextSibling;
    nodes.appendChild(iter);
    iter = next;
}

Expected behavior: next is typed as Node | null.

Actual behavior: next is typed as any, which raises an error under noImplicitAny.

Comment: This is really strange because tooltips indicate that at this point iter is narrowed to Node and iter.nextSibling is of course Node | null.
So it makes no sense that let next = iter.nextSibling isn't type as such.

[jcalz]

Essentially a duplicate of #1146 ?

[jods4]

@jcalz There is a similarity but the reproductions look a bit different (loop vs super usage).
From outside it's not obvious that they are the same bug.
If someone with internal knowledge can confirm it's the same bug, feel free to close this one!

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[jods4]

@mhegazy This was closed as a duplicate of #1146, which it might be, but that issue was closed without resolution...

Is the repro above really not fixable?
Tooltips in VS correctly say iter is Node, so there's no reason that let next = iter.nextSibling isn't typed properly, is there?

[mhegazy]

This is a design limitation of the current compiler implementation. The type of next in the loop depends on the compiler understanding of what the type of iter is, and that in terns depend on type of next. the compiler resolves cycles like such by using any. in theory, yes, the compiler could keep iterating on both types untill a fix point is reached, but this is not how the compiler is currently designed.

[jods4]

@mhegazy OK

I'm still confused by the VS tooltip indicating iter is Node in iter.nextSibling. Where does that information come from? If it's incorrect (compiler sees iter as any) does it imply there's a bug in Typescript tooling inside VS?