More fun with conditional types

conditional.ts

type Rec = {
  'text': string,
  'bool': boolean
};

type Un =
  | {t: 'text', v: string}
  | {t: 'bool', v: boolean};

type Datum<K, V> = { t: K, v: V };
type FromKeys<T, U> = T extends keyof U ? Datum<T, U[T]> : never;
type UnOfRec<T> = FromKeys<keyof T, T>;

type FilterType<T extends {v: any}, K> = (T extends { t: K } ? T['v'] : never);
type RecOfUn<T extends {t: string, v: any}> = {[P in T['t']]: FilterType<T, P>};

// UnOfRec<Rec> = Un
function eq1(x: UnOfRec<Rec>): Un { return x }
function eq2(x: Un): UnOfRec<Rec> { return x }

// RecOfUn<Un> = Rec
function eq3(x: RecOfUn<Un>): Rec { return x }
function eq4(x: Rec): RecOfUn<Un> { return x }