Allow narrowing generic type to single element of union type

It is a fairly common pattern to create a set of generic components, and then to use the generic component type within the system. Currently, typescript has an issue handling cases where some values of a generic component depend on others.

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)
2.4
Code

let index = {
    a: {
        arg: 'a',
        fn: (a: string) => {a.toUpperCase()}
    },
    b: {
        arg: 1,
        fn: (b: number) => {b + 1}
    }
}

let runFn= <K extends keyof (typeof index)>(key: K) => {
    let i = index[key]
    let arg = i.arg
    let fn = i.fn
    fn(arg) // Error: compiler doesn't understand that fn and arg will match
}

Expected behavior:
No error, since we know that arg and fn are pulled off of the same map entry.

Actual behavior:
Error: cannot invoke expression whose type lacks a call signature

Commencez par reproduire l’exemple TypeScript 2.4 fourni et confirmez l’erreur lors de l’appel de fn(arg). Suivez ensuite le chemin de vérification des types pour l’accès indexé via une clé générique et des membres d’union dépendants ; le travail est terminé lorsque l’exemple se compile sans affaiblir la sûreté des types.