9286 - FirstUniqueCharIndex

[kscory]

type Includes<T extends string, U extends string> = T extends `${string}${U}${string}` ? true : false;
type LengthOfString<S extends string, A extends string[] = []> = S extends `${infer U}${infer Rest}`
    ? LengthOfString<Rest, [U, ...A]>
    : A['length'];

type FirstUniqueCharIndex<T extends string, Prefix extends string = ''> = T extends `${infer F}${infer R}`
    ? Includes<`${Prefix}${R}`, F> extends true
        ? FirstUniqueCharIndex<R, `${Prefix}${F}`>
        : LengthOfString<Prefix>
    : -1;