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A regression test is checking causality: Changes in new code, updating dependencies, updating the OS the software is running on, updating shared libraries, porting the code to a new platform, etc. aren't supposed to change the test results.

"I may not know what cos(x) means, but whatever it is shouldn't depend on what OS version I'm running"

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> "I may not know what cos(x) means, but whatever it is shouldn't depend on what OS version I'm running"

Cosine is a terrible example to use for that idea. It's pretty likely to change, for certain x, in similar circumstances to your examples of "when test results should never change".

If it's likely to change, then you especially want the regression test so you can decide how to handle the divergence during your port. Maybe one library preserves the signal on NaNs and the other doesn't. Or maybe the CPU's default rounding mode is different when called in this context, and you're off by 1 ulp.

In either case, if the behavior is to change, it should change as an informed decision and not because nobody noticed.

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