Improve diagnostic when string from property is not assignable to string literal.

TypeScript Version: 3.1.0-dev.20180810

Code

interface IceCreamOptions {
    readonly cherry: "yes" | "no"
}
declare function iceCream(options: IceCreamOptions): void;
const options = { cherry: "yes" };
iceCream(options);

Expected behavior:

Error message recommending to add a type annotation to options. This should be detectable because the source of the failing property is a PropertyAssignment in an object literal being assigned to a variable. (If there is no variable assignment, we could still recommend using ``"yes" as "yes"although that isn't as pretty.) Could also come with a codefix. Note that in JS the fix is slightly different: put/** @type {IceCreamOptions} /` before the `const` or use `/* @type {"yes"} */ ("yes")`.

Actual behavior:

Error messages says Type 'string' is not assignable to type '"yes" | "no"'. with no information on how to fix it.

Commencez par reproduire l’exemple TypeScript 3.1 fourni et retracez le diagnostic d’assignabilité pour l’affectation de la propriété d’un littéral d’objet. Déterminez où le message existant est produit et comment TypeScript identifie la propriété source ; le travail est terminé lorsque le diagnostic recommande une annotation appropriée, avec un codefix uniquement si cette portée est acceptée.