keyof types don't appear to be subtypes of string when used in mapped types

[jtheisen]

TypeScript Version: 2.2.0

Code

function foo<T>() {
    let x: { [P in keyof T]: number }
    let y: { [k: string]: number } = x;
}

Expected behavior:

The assigment should be allowed.

Actual behavior:

The assignment isn't allowed.

This is despite this variant working just fine:

let x: {[P in 'one' | 'two']: number } = null;
let y: { [k: string]: number } = x;

And this works too, as just as the documentation states, keyof T is a subtype of string for all T:

function foo<T>() {
    let kt: keyof T = '' as keyof T;
    let sn: string = kt;
}

[RyanCavanaugh]

See #14548