MergeDeep : fix producing [never] when merging empty tuples.

[taiyakihitotsu]

Close #1200

Test case

I use IsEqual because this passes uncorrectly on editor (it rejects correctly with npm test though).

expectType<{f: [0]}>({} as {f: [never]})

I think this might be related to this tsd issue

Undefined behavior

type A = MergeDeep<[1, 2], string[], {recurseIntoArrays: true, arrayMergeMode: 'replace'}>
// [string, string, ...string[]]
type B = MergeDeep<[1, 2], string[], {recurseIntoArrays: true, arrayMergeMode: 'spread'}> 
// [string, string, ...string[]]
type C = MergeDeep<string[], [1, 2], {recurseIntoArrays: true, arrayMergeMode: 'replace'}> 
// [1, 2, ...string[]]
type D = MergeDeep<string[], [1, 2], {recurseIntoArrays: true, arrayMergeMode: 'spread'}> 
// [1, 2, ...string[]]

This is how it behaves in this PR, but is this according to the spec?
I couldn't find any test cases that verify this, even in the original codebase.

I think this might be somewhat related to this test case, though I'm not certain.

// Should merge array into tuple with object entries
type FooNumberTuple = [Foo[], number[]];
type BarArray2D = Bar[][];

declare const fooNumberTupleBarArray2DSpread: MergeDeep<FooNumberTuple, BarArray2D, {arrayMergeMode: 'spread'; recurseIntoArrays: true}>;
expectType<[FooBarSpread[], Array<number | Bar>, ...BarArray2D]>(fooNumberTupleBarArray2DSpread);

declare const fooNumberTupleBarArray2DReplace: MergeDeep<FooNumberTuple, BarArray2D, {arrayMergeMode: 'replace'; recurseIntoArrays: true}>;
expectType<[FooBarReplace[], Bar[], ...BarArray2D]>(fooNumberTupleBarArray2DReplace);

I'll add A to D test cases if they satisfy the spec.

[som-sm]

I use IsEqual because this passes uncorrectly on editor (it rejects correctly with npm test though).

expectType<{f: [0]}>({} as {f: [never]})

@taiyakihitotsu As long as it works with npm test, that's fine. This is a known limitation with using expectType, you won't get errors in the editor for something like:

expectType<string | number>({} as string);

This behavior already exists in several parts of the codebase, so it's fine to use expectType for tests.

[taiyakihitotsu]

@som-sm

Thanks!

I didn't have a strong conviction regarding the behavior of expectType or how it should be used in type-fest.
That’s why I had been writing the tests via IsEqual (even in my previous PRs)

Now I reverted the tests style.

[sindresorhus]

Suggested change

	expectType<NotEmptyArrayFoo>({} as MergeDeep<NotEmptyArrayFoo, EmptyFoo, {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	expectType<NotEmptyArrayFoo>({} as MergeDeep<NotEmptyArrayFoo, EmptyFoo, {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	expectType<NotEmptyTupleFoo>({} as MergeDeep<EmptyFoo, NotEmptyTupleFoo, {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);
	expectType<NotEmptyTupleFoo>({} as MergeDeep<NotEmptyTupleFoo, EmptyFoo, {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);
	expectType<NotEmptyArrayFoo>({} as MergeDeep<EmptyFoo, NotEmptyArrayFoo, {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);
	expectType<NotEmptyArrayFoo>({} as MergeDeep<NotEmptyArrayFoo, EmptyFoo, {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);

[sindresorhus]

Suggested change

	expectType<{foo: [0, string, {bar: number[]}]}>({} as MergeDeep<NotEmptyArrayFooRecursive, EmptyFooRecursive, {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	expectType<{foo: [0, string, {bar: number[]}]}>({} as MergeDeep<NotEmptyArrayFooRecursive, EmptyFooRecursive, {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	// Tuple and array recursion behavior with primitive values
	expectType<[string, string, ...string[]]>({} as MergeDeep<[1, 2], string[], {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	expectType<[string, string, ...string[]]>({} as MergeDeep<[1, 2], string[], {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);
	expectType<[1, 2, ...string[]]>({} as MergeDeep<string[], [1, 2], {recurseIntoArrays: true; arrayMergeMode: 'replace'}>);
	expectType<[1, 2, ...string[]]>({} as MergeDeep<string[], [1, 2], {recurseIntoArrays: true; arrayMergeMode: 'spread'}>);

[taiyakihitotsu]

@sindresorhus

Thanks for reviews!
Addressed the suggested test cases.