Should strictNullChecks be applicable with Observable filter operator?

[goldenbearkin]

TypeScript Version: 2.0.10

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es5",
        "strictNullChecks": true,
        "noImplicitAny": false,
        "sourceMap": false
    }
}

Code

const mixed = [1, 'one', null, undefined];

const $ = Observable.of(...mixed)
            .filter(x => x !== null)
            .filter(x => x !== undefined)
            .filter(x => typeof x !== 'string');

$.subscribe(x => console.log(x));  //  1

Expected behavior:
$ is in type (number)

Actual behavior:
$ is in type (string | number | null | undefined)

[HerringtonDarkholme]

Related: #9998 (comment)

[mhegazy]

there is not a way to represent this transformation to the type system. an option would be something like #12424

[goldenbearkin]

@mhegazy Could you please explain a little bit more? I still can't get my head around after looking at #12424...