Cohesion Over Testability

A unit test is supposed to verify production code. Sometimes the relationship inverts: the test reaches for an invariant the natural API won't surface, so the production code is split into a "pure inner" and a "thin outer", with the inner exported solely so the test can call it. The split is invisible from the outside (one production consumer, no reuse), but it's permanent shape. The test suite became an architecture client.

That's the smell. The cure is to glue the pieces back into one function and find a different way to pay the regression-coverage bill: integration tests through the real boundary, type-level invariants, or deletion of a test that was insuring trivial branch logic at high seam cost.