@voxpelli/type-helpers/lib/string-types.d.ts

My personal type helpers

export type NonGenericString<T, ErrorMessage extends string = never> =
  string extends T
    // This trick makes it so that the error shown will be that the value didn't match this string literal.
    // With a simple "never" here instead TS gives a sometimes hard to decipher errors
    ? ErrorMessage
    : T;

export type NonGenericStringArray<T, ErrorMessage extends string = never> =
  T extends Array<infer U>
    ? (string extends U ? ErrorMessage : T)
    :
    T extends ReadonlyArray<infer U>
      ? (string extends U ? ErrorMessage : T)
      : T;