That's the whole point of UB though: the programmer helping the compiler do deduce things. It's too much to expect the compiler to understand your whole program to know a+b doesn't overflow. The programmer might understand it doesn't though. The compiler relies on that understanding.
If you don't want it to rely on it insert a check into the program and tell it what to do if the addition overflows. It's not hard.
Whining about UB is like reading Shakespeare to your dog and complaining it doesn't follow. It's not that smart. You are though. If you want it to check for an overflow or whatever there is a one liner to do it. Just insert it into your code.
No, the whole (entire, exclusive of that) point of undefined behaviour is to allow legitimate compilers to generate sensible and idiomatic code for whichever target architechture they're compiling for. Eg, a pointer dereference can just be `ld r1 [r0]` or `st [r0] r1`, without paying any attention to the possibility that the pointer (r0) might be null, or that there might be memory-mapped IO registers at address zero that a read or write could have catastrophic effects on.
It is not a licence to go actively searching for unrelated things that the compiler can go out of its way to break under the pretense that the standard technically doesn't explicitly prohibit a null pointer dereference from setting the pointer to a non-null (but magically still zero) value.
If you don't want it to rely on it insert a check into the program and tell it what to do if the addition overflows. It's not hard.
Whining about UB is like reading Shakespeare to your dog and complaining it doesn't follow. It's not that smart. You are though. If you want it to check for an overflow or whatever there is a one liner to do it. Just insert it into your code.