Type inference for keys in for...in loops

[srolel]

TypeScript Version: 2.1.4

Code

const obj = {x: 1, y: 2};
for (let k in obj) {
   const prop = obj[k];
}

Expected behavior:

let k: "x" | "y";
let prop: number;

Actual behavior:

let k: string;
let props: any;

[ahejlsberg]

Duplicate of #12314.