use conditional type inference to "copy" a type T into a new type variable O and then an identity-like mapped type which iterates through the copied type's properties. The conditional type inference is conceptually a no-op, but it's used to distribute union types and to force the compiler to evaluate the "true" branch of the conditional (if you …

expand.ts

// https://stackoverflow.com/a/69288824

export type Expand<T> = T extends (...args: infer A) => infer R
  ? (...args: Expand<A>) => Expand<R>
  : T extends infer O
  ? { [K in keyof O]: O[K] }
  : never;

export type ExpandRecursively<T> = T extends (...args: infer A) => infer R
  ? (...args: ExpandRecursively<A>) => ExpandRecursively<R>
  : T extends object
  ? T extends infer O
    ? { [K in keyof O]: ExpandRecursively<O[K]> }
    : never
  : T;