Provide a no-parameter creator when type is "empty"

[tvald]

For "empty" types ({}, void, null), the Creator function no longer accepts an argument.

The spec for whether a type is empty is as follows:

// "T" (parameter required)
type T1 = ObjectHasKeys<1>
type T2 = ObjectHasKeys<''>
type T3 = ObjectHasKeys<true>
type T4 = ObjectHasKeys<symbol>
type T5 = ObjectHasKeys<{ foo: string }>
type T6 = ObjectHasKeys<{ foo?: string }>

// "F" (parameter not accepted)
type F1 = ObjectHasKeys<{}>
type F2 = ObjectHasKeys<null>
type F3 = ObjectHasKeys<void>

[coveralls]

Coverage remained the same at 100.0% when pulling c2e24c0 on tvald-contrib:feature/nokey-types into f3cede5 on pelotom:master.

[pelotom]

This will be a great improvement 👍

[pelotom]

How about just using typelevel-ts for this machinery? There's really no downside since it's purely a static dependency.

[tvald]

This is actually different from typelevel-ts's implementation of StringContains, which fails when applied to the keys of a primitive like number or boolean.

I can look into trying to work around that failure, or perhaps getting a PR into typelevel-ts, since it would definitely be preferable to depend on an external library for all of this.

[pelotom]

I see. Well, no need to go into contortions for it.

[pelotom]

Hm, maybe it would be better to only apply this trick when the union's value type is precisely undefined? Otherwise you can do this:

const { value } = unionize({ foo: ofType<null>() }, 'tag', 'value').foo()

The type system will infer value: null, but its value will actually be undefined... similar problem for ofType<{}>(). All we really need is one special nullish type which enables the no-arg creation function.

[pelotom]

Also could you add a test for this?

[tvald]

Closing this in favor of an implementation based on conditional types, since they'll provide far more flexibility for introducing type-dependent modifications of the unionize API.