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.

ES2016 の String のライブラリ宣言と、既存の Uppercase および Lowercase intrinsic types から始めます。サンプルを要求された戻り値の型と照合してから、ロケールのバリアントに個別の扱いが必要かどうかを調査します。完了条件は、string の動作を変更せずにリテラル文字列のケースが表現されていることです。