PR Checklist

• Addresses an existing open issue: related to #000
• Steps in Contributing were taken

Overview

Currently, IsTuple doesn’t recognise arrays with a rest element and optional elements as tuples, such as [number?, …string[]].

import { IsTuple } from 'ts-essentials';

type T = IsTuple<[number?, ...string[]]>;
//   ^? type T = never

This PR fixes that by updating the implementation of IsTuple.

However, the above fix causes the following test to fail in DeepNullable (similar with DeepUndefinable):

Assert<IsExact<DeepNullable<never[]>, null[]>>

There are two problems here:

1. Incorrect behavior in IsTuple for never[]:
   Previously, IsTuple<never[]> returned never[], which is incorrect—it should return never. The updated implementation corrects this.

2. Incorrect handling of never in DeepNullable:
   Once IsTuple<never[]> correctly returns never, DeepNullable<never[]> starts returning never[] instead of null[]. This happens because DeepNullable<never> currently returns never.

   The issue becomes more apparent when testing with a case that doesn't involve IsTuple:

   type T = DeepNullable<{ foo: never }>;
   //    ^? { foo: never } (expected: { foo: null })

   To fix this, this PR makes DeepNullable<never> return null.