feat: respect nullable() with oneOf

[jquense]

closes: #768 #104

BREAKING CHANGE: previously oneOf required adding null explicitly to allowed values when using oneOf. Folks have found this confusing and unintuitive so I am deferring and adjusting the behavior

[karlhorky]

Thanks for this @jquense !

How do you expect users to represent "only null" using mixed() now? My previous solution here does not seem to work anymore:

  }).shape({
    files: mixed()
      .label('Files')
      .oneOf([null], ({ label }) => `${label} must be null`)
      .nullable()
      .defined(),
  });

Error message:

Type 'null' is not assignable to type 'Reference<unknown> | AnyPresentValue | undefined'.

[karlhorky]

Oh, maybe just moving .nullable() up one line? This seems to not fail:

  }).shape({
    files: mixed()
      .label('Files')
+     .nullable()
      .oneOf([null], ({ label }) => `${label} must be null`)
-     .nullable()
      .defined(),
  });

[jquense]

simplest way to do this is const nullSchema = mixed((value) === null)

[karlhorky]

Hm, do you mean the following with a type predicate? This seems to throw a type error.

mixed((value): value is null => value === null) // 💥 Type 'null' is not assignable to type 'AnyPresentValue'.
  .nullable();

Full TypeScript error:

Argument of type '(value: any) => value is null' is not assignable to parameter of type 'MixedOptions<AnyPresentValue> | TypeGuard<AnyPresentValue> | undefined'.
  Type '(value: any) => value is null' is not assignable to type 'TypeGuard<AnyPresentValue>'.
    Type predicate 'value is null' is not assignable to 'value is AnyPresentValue'.
      Type 'null' is not assignable to type 'AnyPresentValue'.

Also does not work if passing in the generic parameter:

mixed<null>((value): value is null => value === null) // 💥 Type 'null' does not satisfy the constraint 'AnyPresentValue'.
  .nullable();

---

Once we have a recommended version, I'll add this to the readme as an example for mixed()