interface Baz {
  foo: { a: number };
}

const createDefaultExample = <K extends keyof Baz>(x: K): Baz[K] & { x: K; } => {
  return { a: 0, x }; // okay in TS4.7, error in TS4.8
}

type Foo<K> = K extends unknown ? { a: number } : unknown
const createDefaultExample = <K,>(x: K): Foo<K> & { x: K; } => {
  return { a: 1, x: x }; // okay in TS 4.7.4, error in TS 4.8.2
}