"Null" coalescing type unable to be inferred

http://en.wikipedia.org/wiki/Null_coalescing_operator#JavaScript

``` javascript
/** @type {!Window|!Document} */
var x;
/** @type {!Document} */
var y = x instanceof Window ? x.document : x;
/** @type {!Document} */
var z = (x instanceof Window && x.document || x);
```

with `--jscomp_error=checkTypes` generates:

```
test.js:6: ERROR - initializing variable
found : (Document|Window)
required: Document
var z = (x instanceof Window && x.document || x);
^

1 error(s), 0 warning(s), 100.0% typed
```

`y` is inferred correctly whereas `z` not.