type UndefinedKeys<T> = { [P in keyof T]: undefined extends T[P] ? P : never }[keyof T]
type OptionalUndefined<T> = Partial<Pick<T, UndefinedKeys<T>>> & Omit<T, UndefinedKeys<T>>

type UndefinedKeys<T> = { [P in keyof T]: undefined extends T[P] ? P : never }[keyof T]
type OptionalUndefined<T> = Partial<Pick<T, UndefinedKeys<T>>> & Omit<T, UndefinedKeys<T>>

type TSample<
  U extends Record<string, any> | undefined,
  D extends Record<string, any>
> = { undef: U, def: D }


function fn<
  U extends Record<string, any> | undefined,
  D extends Record<string, any>
>(params: OptionalUndefined<TSample<U, D>>) {
  {
    // ERROR: both properties do not exist on type 'OptionalUndefined<TSample<U, D>>'
    const { def, undef } = params
  }

  {
  // works as expected
  const { def, undef } = params as OptionalUndefined<
      TSample<
        Record<string, any> | undefined,
        Record<string, any>
      >
    > 
  }
}