Property based testing is good at generating boundary values for inputs. But for any more complicated piece of code getting boundary value coverage of interior values is an open problem that requires instrumentation feedback to understand branch coverage and value coverage of the code that got tested. It’s not an easy thing at all.

> The semantics (or lack thereof) of C and C++ can make this difficult to actually test for because the compiler is allowed to say "test passed" to any input leading to UB.

I get what you are saying but does this actually apply to a test? If the code under test is in one compilation unit and the test harness in another and they are linked together then the UB optimization issue ends at the API boundary and can't possibly make the test pass ..?