Type Alias At<A, N, Fail>

At: ArrayPlus.IndexAt<A, N, Fail, Fail, Fail> extends infer I
    ? I extends number
        ? IsTuple.$<
            A,
            {
                $else: A[I]
                | undefined;
                $then: NumberPlus.IsNumber.$<
                    I,
                    { $else: A[I]; $then: A[I]
                    | undefined; exact: true },
                >;
            },
        >
        : Fail
    : never

type R = At<[1, 2, 3], 2> // 3
type R = At<[1, 2, 3], -1> // 3
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