I’ve read the readme a few times and still have no idea how this is useful. The motivating example of Sort => SortBy has no description of what’s highlighted and no description of how. Is it running the old test suite with the new changes as a weak form of mutation testing? Something else?
Not mutation testing — nothing is mutated. It builds both branches with instrumentation, runs the same test suite on each, and diffs the recorded arguments and return values.
In that example, selectDiscount returns a different discount and the order total goes 85 → 60. Neither function is in the diff; only the sort helper is. The tests still pass — one asserts the total is positive, and 60 is positive.
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