type Pizza = { type: 'pizza'; topping: string };
type Sandwich = { type: 'sandwich'; condiments: string[] };
type Food = Pizza | Sandwich;

const fn = (food: Pizza | Sandwich) => {
    if (isMatching({ type: 'oops' }, food)) {
        //                  👆 used to type-check, now doesn't!
    }
}

const getWithDefault = <T extends P.Pattern>(
  input: unknown,
  pattern: T,
  defaultValue: P.infer<T> //  👈
): P.infer<T> =>
  isMatching(pattern, input) ? input : defaultValue

const res = getWithDefault(null, { x: P.string }, 'oops') 
//                                     👆           👆 type error should be here
//                                 but it's here 😬