String case change methods should return intrinsic string manipulation types

lib Update Request

Configuration Check

My compilation target is ES2018 and my lib is ["ES2016", "dom"].

Missing / Incorrect Imprecise Definitions

At least toUpperCase and toLowerCase methods on String, maybe/probably also the Locale versions?

Sample Code

type LittleT = "t" | "tt";
type BigT = Uppercase<LittleT>;
declare const smol: LittleT;
const big: BigT = smol.toUpperCase(); // err, string is not assignable to "T" | "TT

Documentation Link

https://tc39.es/ecma262/#sec-string.prototype.tolowercase

Note

I'm not sure about the Locale versions of these methods because I don't know what algorithm the "intrinsic" Uppercase<T> / Lowercase<T> helper types are required to follow. (Do we need separate LocaleUppercase / LocaleLowercase helpers?)

Also worth mentioning: I think what I'm looking for is a return type of e.g. Uppercase<this>, which shouldn't have an impact on non-literal string types because Uppercase<string> is just string.

Commencez par les déclarations de bibliothèque ES2016 pour String et les types intrinsèques existants Uppercase et Lowercase. Vérifiez l’exemple par rapport aux types de retour demandés, puis examinez si les variantes de locale nécessitent une gestion séparée ; le travail est considéré comme terminé lorsque le cas de chaîne littérale est représenté sans modifier le comportement de string.