[AlexDenisov]

This is a valid concern, for sure. In fact, these are called Equivalent Mutants. We observed it several times, here are some examples. You flip the if/then branches and the code essentially does the same, switching from a fast to slow implementation of the same algorithm. Another example is mutating the code in a C++ destructor, which is not easy to test.

With that being said, even despite the equivalent mutants, you can get a pretty good insight. Here is an example of finding from a proprietory real-time OS used in the space industry: https://gist.github.com/AlexDenisov/b5d2e23457b88813b5ab9d5d... (this is a part of an email which I could safely post online).