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Narrow String#toLowerCase/toUpperCase return types via Lowercase/Uppercase#9
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…rcase Make String.prototype.toLowerCase and toUpperCase preserve string-literal types in their return type, by typing them as `<T extends string>(this: T): Lowercase<T>` and `Uppercase<T>` respectively. For non-literal `string` receivers, `Lowercase<string>` resolves to `string`, preserving existing behavior. For literal receivers (e.g. `"FOO".toLowerCase()`), the result narrows to the corresponding literal (`"foo"`). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Originally PR microsoft#63460 in microsoft/TypeScript by @ljharb