SelectArray type

If TFields is never, an untyped string array Otherwise, a narrowed Fields[] type to be used elsewhere in the consuming type.

type SelectArray<TFields> = [string] extends [TFields]
  ? readonly TFields[]
  : () => (T extends TFields ? true : false) extends () => T extends never
      ? true
      : false
      ? readonly string[]
      : readonly TFields[]