Oh thank God! I have been asking for something like "erroneous behaviour" for years. Clang in particular is very fond of deleting safety checks if they check fo

Oh thank God! I have been asking for something like "erroneous behaviour" for years. Clang in particular is very fond of deleting safety checks if they check for conditions which it thinks are only possible under undefined behaviour. Every year I have to fix one or two bugs that stem from code from completely separate modules interacting in a way that manages to produce a state which leads to undefined behaviour. The option to opt-in to UB is also a very good idea - we can turn it on in the hot parts of code and restore performance where it really matters, while constraining the impact, i.e. even if the code has UB, that cannot poison the rest of the program. I hope this renewed focus on safety will lead to the default for what is currently undefined behaviour being more in line with what was originally intended - sort-of arbitrary behaviour dependent on the platform's state at the time of code execution. As in, less constrained than "implementation defined", since there is no guarantee it will produce the same result every time, but more constrained than UB, since it does not invalidate the program.