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The lesson here is that if a feature (at a minimum) does not have a associated test case, it is not actually a feature.

Yes, I agree. I find the addition of the regression test the true long-term fix. The code is just an opaque incantation that may or may not preserve some property we find worth preserving and we have no way of knowing it keeps preserving it over time as other parts of the system change.

The test actually proves it and while it too can change it has more staying power because it's expressed at a higher level of abstraction ("random arcane weird C shit" in the case of code versus "does this property hold" in the case of a regression test).

I have not looked into this specific issue, but are we sure that a regression here could have been avoided via a localized test? Maybe issues seem to arise from A implementing a feature with tests. B seeing that A lacks some functionality and adding it (potentially with tests), C seeing this (extra) functionality in A, and using in unintended ways not covered by tests (or in an unintended environment) + multiply by many layers of this A-B-C story up and down the stack.

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