pick-union-string.ts

type _Size = "x-small" | "small" | "medium" | "large" | "x-large";

type USToI<T extends string> = {
  [P in T]: unknown;
};

export type Size<K extends keyof USToI<_Size>> = keyof Pick<USToI<_Size>, K>;
  
// Error!
// const size: Size<"small" | "large"> = "medium";