ArrayTail: Fix support for optional parameters

[ferferga]

Fixes #978

[som-sm]

@ferferga Following solution addresses the issue mentioned in #978 while ensuring all existing tests continue to pass.

export type ArrayTail<TArray extends UnknownArrayOrTuple> = keyof TArray & `${number}` extends never
	? []
	: TArray extends readonly [unknown?, ...infer Tail]
		? Tail
		: [];

The conditional keyof TArray & `${number}` extends never is to separate tuples from empty arrays and non-tuple arrays like string[], so that we can handle them independently.


However, the updated implementation fails when the type is instantiated with unions. For example, TArray<[1, 2] | []> now incorrectly resolves to [] instead of the expected [2] | [] (although currently there’s no test covering this behaviour). To ensure the type continues to work correctly with unions, we can simply reorder the conditionals.

export type ArrayTail<TArray extends UnknownArrayOrTuple> = TArray extends readonly [unknown?, ...infer Tail]
	? keyof TArray & `${number}` extends never
		? []
		: Tail
	: [];

[ferferga]

@som-sm Thank you very much for your input, works flawlessly! Added you as a co-author and took the chance to also add some tests.