It is a bug - you’ve violated the contract between the language and the compiler.
Just like segfault or logic bug, it’s a class of bugs. Why is special though is that in most bugs you just hit an invalid state. In UB you can end up executing code that never existed or not executing code that does exist. Or any number of other things can happen because the compiler applies an optimization assuming a runtime state you promised it would never occur but did.
It’s slightly different from being a strict subset because UB is actually exploited to perform optimizations - UB is not allowed so the compiler is able to emit more efficient code is taught to exploit that and the language allows for it (eg the niche optimization the blog describes)
Just like segfault or logic bug, it’s a class of bugs. Why is special though is that in most bugs you just hit an invalid state. In UB you can end up executing code that never existed or not executing code that does exist. Or any number of other things can happen because the compiler applies an optimization assuming a runtime state you promised it would never occur but did.
It’s slightly different from being a strict subset because UB is actually exploited to perform optimizations - UB is not allowed so the compiler is able to emit more efficient code is taught to exploit that and the language allows for it (eg the niche optimization the blog describes)