If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation.
(copied from a comment on blog.regehr.org)
This is actually a really common area of misunderstanding. But ultimately, one of the main uses of unions is to provide multiple ways to access the same data.
where answers try to distill the standard into something legible by humans. Unsurprisingly, they come to different conclusions.
The overall interpretation in some situations, if you look at things just right, you can do type punning in C++. But it's pretty risky since you're a LOC away from the compiler deleting your function.
Or, you can't really do it at all safely. Which I would conclude from the above anyways.
As for integer overflow at least gcc and clang have built-in functions that generate optimal code by checking the CPU's overflow flag: http://clang.llvm.org/docs/LanguageExtensions.html#checked-a... and https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins...