No more dangerous than existing UB in C, just that we're not used to it. And it's easy to disable for your types.
Like Steve said, the exact nature of this UB isn't completely specified yet (as in, when it actually is UB), so it is dangerous right now, but that's just temporary.
Like Steve said, the exact nature of this UB isn't completely specified yet (as in, when it actually is UB), so it is dangerous right now, but that's just temporary.