Maintain string litteral types when used as index signature type

[mohsen1]

TypeScript Version: 2.4.0

Code

function makeObj(foo: 'FOO') {
    return { [foo]: 1 };
}

makeObj('FOO') // return type: `{ [x: string]: number}`

Expected behavior:
Index signatures should have narrower types. Return type should be { [x: 'FOO']: number}

Actual behavior:
return type is: { [x: string]: number}

Use case

When making forms in React components I often use a method that returns an onChange method to be used on input onChange properties:

class Component<S> {
    state: S;
    setState(partialState: Partial<S>) { }
    onFiledChanged = (filedName: keyof S) => (value) => {
        this.setState({ [filedName]: value });
    }
    render() {
        return <form>
                <input onChange={ this.onFiledChanged('foo') } />
                <input onChange={ this.onFiledChanged('bar') } />
            </form>
    }
}

TypeScript fails to understand filedName is type 'foo' | 'bar'

[mhegazy]

looks like a duplicate of #16687

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.