For inputs containing only optional elements, I originally wasn't splitting the result, for example, ArrayReverse<[1?, 2?, 3?]> was returning [3?, 2?, 1?]. While this type of output is accurate, it's not precise, because [3?, 2?, 1?] could be [3] | [3, 2] but reversing [1?, 2?, 3?] would never yield [3] or [3, 2].

@benzaria Would appreciate if you could share your thoughts on this!

@sindresorhus While adding test for long tuples, I realised my implementation was not tail recursive because I had simplified a conditional using the If type. While this mistake is obvious, it might get missed. Refer #6cc3e53.

So, does it make sense to add a note like the following to the If type or is it too trivial?

Note: Sometimes using the If type can make an implementation non tail recursive, which would affect the performance of the implementation. In such cases, it's better to use a conditional directly. Refer the following example:

import type {If, IsEqual, StringRepeat} from 'type-fest';

// The following implementation is not tail recursive
type Includes<S extends string, Char extends string> =
	S extends `${infer First}${infer Rest}`
		? If<IsEqual<First, Char>,
			'found',
			Includes<Rest, Char>>
		: 'not found';

// Hence, instantiations with long strings will fail
type HundredZeroes = StringRepeat<'0', 100>;

// @ts-expect-error
type Fails = Includes<HundredZeroes, '1'>;
//           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Error: Type instantiation is excessively deep and possibly infinite.

// However, if we use a simple conditional instead of `If`, the implementation becomes tail-recursive
type IncludesWithoutIf<S extends string, Char extends string> =
	S extends `${infer First}${infer Rest}`
		? IsEqual<First, Char> extends true
			? 'found'
			: IncludesWithoutIf<Rest, Char>
		: 'not found';

// Now, instantiations with long strings will work
type Works = IncludesWithoutIf<HundredZeroes, '1'>;
//=> 'not found'

So, does it make sense to add a note like the following to the If type or is it too trivial?

Definitely makes sense 👍

Is this ready?

Is this ready?

@sindresorhus Yeah, it's ready!

I think we can proceed with this PR and close out #1167, since it still has some unresolved issues. This PR already credits it, so it should be fine to move forward.