Add UnionMember type

[taiyakihitotsu]

Add UnionMember

• This is useful to recurse union-types, so I think this should be exported as a global for end-users.
• We can evade a duplicated definition potentially.

And the current UnionMember, which locally defined in UnionToTuple, returns unknown.

expectType<never>({} as UnionMember<never>);
//=> unknown

This would cause an error if end-users use IsNever for termination check.

Ideally, UnionMember reduces a type, e.g., assuming $A$ and $B$ are type arguments of UnionMember<A | B>, $A \cup B$ -> $B$, so $\emptyset$ -> $\emptyset$ intuitively ($\bot$ -> $\bot$).
This PR's UnionMember handles this.

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Note

• This PR is separated from UnionToTuple: Fix behavior when a union member is a supertype of another; Add ExcludeExactly type #1349

[taiyakihitotsu]

@sindresorhus

Thanks for your reviews!

And I believe this suggestion implies that test-d/last-of-union.ts should have a test for recursion without directly importing UnionToTuple to clarify it. Right?

If so, UnionToTuple in test-d/last-of-union.ts fulfills the requirement. Would it be okay to integrate both in test-d/last-of-union.ts?

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#1368 (comment)

type UnionToTupleWithExclude<T, L = LastOfUnion<T>> =
	IsNever<T> extends false
		? [...UnionToTupleWithExclude<Exclude<T, L>>, L]
		: [];

expectType<1 | 2 | 3>({} as UnionToTupleWithExclude<1 | 2 | 3>[number]);

[som-sm]

PR mostly LGTM, just few minor comments.

[taiyakihitotsu]

@som-sm

Thank you!
Picked your comment: #1368 (comment)

I guess that's why you chose not to add an example.

Yes.
Now I think UnionLength defined in the doc is enough to describe how to write a recursion of union type.

[som-sm]

@sindresorhus If this LGTY, this can be merged. Also, #1349.