type Input = { type?: 'one' } | { type: 'two' };

const f1 = (input: Input) =>
  match(input)
    .with({ type: 'one' }, () => {})
    .with({ type: 'two' }, () => {})
    .exhaustive(); // shouldn't type-check, but does 👎


const f2 = (input: Input) =>
  match(input)
    .with({ type: 'one' }, () => {})
    .with({ type: 'two' }, () => {})
    .with({ type: undefined }, () => {}) // <- the type key needs to be present.
    .exhaustive();  // shouldn't type-check, but does 👎

type Input = { type?: 'one' } | { type: 'two' };

const f = (input: Input) =>
  match(input)
    .with({ type: 'one' }, () => {})
    .with({ type: 'two' }, () => {})
    .with({ type: P.optional(undefined) }, () => {}) // <- the type property may not be there
    .exhaustive(); // ✅