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)

提供された for...in と typeof の例で TypeScript 2.1 のエラーを再現し、その後 TypeScript 2.0 での動作と比較します。for...in の narrowing パスを通じた this[a] の型推論を追跡します。明示的な cast なしで例の型チェックが通り、期待される関数の narrowing が維持されれば完了です。