`Merge`: Ensure idempotency so `Merge<A, A>` returns `A` #1336

Merged

sindresorhus merged 16 commits intosindresorhus:mainfrom

taiyakihitotsu:fix/fixed-length-array/20260127

Jan 31, 2026

Commits on Jan 27, 2026

refactor: `IsEqual`, remove unused import, shorten definition
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fix: `FixedLengthArray`, keep idempotency
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test: check idempotency of `Merge`, ensure to keep branded type in `IsEqual`
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test: add `Merge` test, ensure idempotency of `FixedLengthArray`
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fix: `IsEqual`, intersection objects are equivalent to merged objects
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fix: `Merge`, return arg if both are equal

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fix: `Merge` returns `Simplify<Destination>` if both args are equal
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test: `Merge`, add tests to ensure idempotency and assinability
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revert: `IsEqual`
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revert: `FixedLengthArray`
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fix: remove `Simplify` from `Merge`, ensure union distribution

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