/** Returns true if T represents a single string literal and not a union or `string`. */
export type IsSingleStringLiteral<T extends string> = string extends T
  ? false
  : IsSingleStringLiteralHelper<T> extends never
    ? true
    : false;

type IsSingleStringLiteralHelper<T, TCopy = T> = T extends string
  ? Exclude<TCopy, T>
  : never;