for..in variable does not have the type keyof T

[mhegazy]

function set<T, K extends keyof T>(obj: T, key: K, value: T[K]) {
    obj[key] = value;
}

var O = { a: "string" };

// var k: keyof typeof O;
for (var k in O) {
    set(O, k, "another"); // string is not assignable to "a"
}

[ahejlsberg]

I have my doubts about this one. In for (var k in x) where x is of some type T, it is only safe to say that k is of type keyof T when the exact type of x is T. If the actual type of x is a subtype of T, as is permitted by our assignment compatibility rules, you will see values in k that are not of type keyof T.

The same argument holds for the built-in Object.keys() method.