Narrow "this" type

TypeScript Version: nightly (2.2.0-dev.20161130)

Code

function f(this: string | number) {
    if (typeof this === "string") {
        const x: string = this;
    }
}

Expected behavior:

No error.

Actual behavior:

a.ts(3,15): error TS2322: Type 'string | number' is not assignable to type 'string'.

Rename this to me and it works.

[yume-chan]

class A {
    doA() { }

    tryDoB() {
        if (isB(this)) {
            this.doB(); // works
        }
    }
}

class B extends A {
    doB() { }
}

declare function isB(value: any): value is B;

function tryDoB(this: A): void {
    if (isB(this)) {
        this.doB(); // Property 'doB' does not exist on type 'A'.
    }
}

Type guards work on this of instance methods, but not normal functions, it's so annoying.