It “working” isn’t the problem, the problem is when the compiler makes use of the “this is UB” to make formerly correct code incorrect.
The definition of UB would allow a type punned union on the stack for instance to be treated as logically a struct - eg changes to the float value would not impact the integer value. While I agree that would not happen in practice today there is no reason it has to continue to work.
The fact that compilers already make sane decisions regarding some UB - evaluation order of parameters used to be unspecified and gcc at least would change evaluation order depending of optimisation level, but all compilers have realized that just insane and so left to right evaluation. Similarly bzero will initialise padding bytes in a struct even though that is technically UB, so could technically be skipped.
Sorry for run on horror sentences I’m typing on my phone :)
The definition of UB would allow a type punned union on the stack for instance to be treated as logically a struct - eg changes to the float value would not impact the integer value. While I agree that would not happen in practice today there is no reason it has to continue to work.
The fact that compilers already make sane decisions regarding some UB - evaluation order of parameters used to be unspecified and gcc at least would change evaluation order depending of optimisation level, but all compilers have realized that just insane and so left to right evaluation. Similarly bzero will initialise padding bytes in a struct even though that is technically UB, so could technically be skipped.
Sorry for run on horror sentences I’m typing on my phone :)