Type Inference in for... in combined with typeof

TypeScript Version: 2.1.4

The following code gives an error in TS 2.1:
It's not really nice code, but I think it should work never the less :)

for (var a in this) {
            if (typeof this[a] === "function" && !ko.isObservable(this[a]))
                this[a] = this[a].bind(this);
        }

Expected behavior:
I would expect this[a] to be typed as this[keyof this] & Function:

for (var a in this) {
            if (typeof this[a] === "function" && !ko.isObservable(this[a]))
                this[a] =( <this[keyof this] & Function> this[a]).bind(this);
        }

Actual behavior:
It gives an error, as of TS 2.1, it worked in TS 2.0 (because mapped types did not exist, this[a] was typed as any)

Reproduce the TypeScript 2.1 error with the provided for...in and typeof example, then compare its behavior with TypeScript 2.0. Trace type inference for this[a] through the for...in narrowing path; done means the example type-checks without the explicit cast while preserving the expected function narrowing.