Type Alias PathValue<T, P>

PathValue: string extends P
    ? unknown
    : P extends `${infer S}.${infer Rest}`
        ? PathValue<NonNullable<Seg<T, S>>, Rest>
        : Seg<T, P>

The type at patch path P within T (paths as in PatchEntry: dot-separated keys with [n] indexes, negative-from-end allowed).