What I don't get is, why are we still adding more undefined behaviour to the standard? Because it allows flexibility for compilers to implement features effici

What I don't get is, why are we still adding more undefined behaviour to the standard? Because it allows flexibility for compilers to implement features efficiently on various platforms. You can define undefined behavior if you wish. Make your own unreachable that prints an error and aborts gracefully when reached in debug builds, and calls std::unreachable in release builds.