// A *self-contained* demonstration of the problem follows...
function f<T, K extends keyof T>(obj: T, k: K, value:T[K]) {
  for (let key in obj) {
    value = obj[key];
    break;    
  } 
  return value;
}

f({ x: 1, y: "" }, 'y', "1").charAt(0); //runtime error