No excess property checks for nullary sums

[Magellol]

The following produces a runtime exception with no type errors.

import { getShow } from '@unsplash/sum-types-fp-ts';
import * as Sum from '@unsplash/sum-types';

type S = Sum.Member<'A'> | Sum.Member<'B'>;
const S = Sum.create<S>();

const show = getShow<S>({
  A: 'Custom',
});

console.log(show.show(S.mk.A));

https://stackblitz.com/edit/typescript-5afkt4?file=index.ts

I believe if supplied, A must be an instance of Show but TS doesn't check for it.

Side note, I find it a little confusing prepending the sum member key in the show output here https://github.com/unsplash/sum-types-fp-ts/blob/master/src/index.ts#L153 . In the following code I'm expecting the show output to be CustomA but instead I get A CustomerA

const show = getShow<S>({
  A: {
    show: () => "CustomA"
  }  
});

[samhh]

This appears to only be an issue with nullary sums. Add any non-nullary member and A will be flagged as an unknown property.

Looks like an issue with {} as well:

// no error
const x: {} = {
  B: 123,
}

// unknown property error on `B`
const y: { A: string } = {
  B: 123,
}

That looks like expected (questionable) TS behaviour: microsoft/TypeScript#47486 (comment)

This bug very likely applies to the other exports as well, both here and in other libraries.