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.

Inizia con le dichiarazioni della libreria ES2016 per String e i tipi intrinseci esistenti Uppercase e Lowercase. Verifica l’esempio rispetto ai tipi restituiti richiesti, quindi analizza se le varianti di locale richiedono una gestione separata; il lavoro è completato quando il caso di stringa letterale è rappresentato senza modificare il comportamento di string.