Type Alias SelectKeys<T, U, Mode>

SelectKeys: {
    [P in keyof T]-?: Mode extends "normal"
        ? T[P] extends U ? P : never
        : U extends T[P] ? P : never
}[keyof T]

From T, construct a union of the keys which are assignable to U

T
U
Mode extends "normal" | "invert" = "normal"
If set to 'invert', will use keys to which U is assignable