@sindresorhus why was readonly [] chosen here? I'm unable to create a castArray function that is compatible with this.

https://www.typescriptlang.org/play/?#code/JYWwDg9gTgLgBAbwIJSgQwJ5oEYBsCmAvnAGZQQhwDkMGY+AtCfgM4xUDcAUFyQK4A7AMYxgEAXCFo2KdBgCMAHgCSMfCAB8ACgBuaXH3wAuOLMw4CKtZoCUJ1eoDaAXURcAkFHww+UCWYwAOmAWAN19Qxs4AH44PQN8OBNHeMNnbkIefmFRcUlpGACAJit1bVTjU1RzPHxS2xMvNAATcVwMOAcQFzdPb19-aqCQsIqo2IqkuBSI-HSuTKA

@fregante Item cannot be an array type, because Arrayable<unknown[]> is equivalent to unknown[] | readonly unknown[][], NOT unknown[] or readonly unknown[]. If Item is an Array type, Array.isArray will always return true, meaning that the actual return type of castArray1 / castArray2 is actually Item itself, not Item[].
I'm guessing what you're looking for is something like:

import { Arrayable } from 'type-fest';

type Primitive =
	| bigint
	| boolean
	| null
	| number
	| string
	| symbol
	| undefined;

function castArray<Item extends Primitive | Record<PropertyKey, unknown>>(value: Arrayable<Item>): Item[] {
	return Array.isArray(value) ? value : [value];
}

My previous code worked correctly if readonly is dropped, so that lengthy type information is a consequence of this keyword.

You can see it working here: https://www.typescriptlang.org/play/?#code/JYWwDg9gTgLgBAbwIJSgQwJ5oEYBsCmAvnAGZQQhwDkMGY+AtCfgM4xUDcAUF7fXAAVcaYADsU6LHnwAeABoA+OAF44cuAB81AbQC63HiQCuogMYxgEUXFNo2EzAEYZASRj4QCgBQA3NLiN8AC5BYTEHKQJXd08AShC3Dz1ELgBIKHwYIyhrCIA6YBYI339A2LgAfjg-APw4EO0awP0uQkMTc0trW3tUTAAmaI9vJuC4CJwoxLiEmOSENIysnPG+jAKitZLa8qrR+rhG0vwWtqA

In your case, it doesn't even support regular class instances, like castArray(new Date())