Destructuring assignment doesn't narrow undefined, inconsistent with regular property access #10846

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).