Destructuring assignment doesn't narrow undefined, inconsistent with regular property access

[Arnavion]

TypeScript Version: 2.1.0-dev.20160910

Code

declare const foo: { bar: string | undefined };

if (foo.bar !== undefined) {
    foo.bar.length; // Property access (1)
}

if (foo.bar !== undefined) {
    const bar = foo.bar; // Property access
    bar.length; // (2)
}

if (foo.bar !== undefined) {
    const { bar } = foo; // Destructuring assignment
    bar.length; // (3)
}

tsc -t es5 -m amd --strictNullChecks ./foo.ts

Expected behavior:

All three compile, or all three complain that .length is being invoked on string | undefined

Actual behavior:

Only (3) complains that error TS2532: Object is possibly 'undefined'. In (1) foo.bar is inferred as string, and thus in (2) bar is also inferred as string. But in (3) bar is string | undefined

---

I'm actually surprised that a check on foo.bar narrows it down within the if blocks at all, since I remember this only used to narrow bindings. I guess this changed at some point to support narrowing properties, but then it should also narrow (3) identically to (2).

[mhegazy]

This is essentially the same as #10065. control flow analysis only narrows the type of the right most symbol. in this case foo.bar; it does not narrow the type of foo to {bar: string}. the first two cases you check on foo.bar and you use foo.bar so no issues, in the third you are looking at the type of foo. since it was not narrowed, you get the original type, and thus bar is still string | undefined.

closing as a duplicate of #10065