the placement new is not UB, at least at a language level. Modifying the vector would break the vector invariant, leading to UB. The specialization is UB only because vector is an std type, otherwise is perfectly legal.

In both cases, yes, if you hardcode the specific std::vector invariants in the compiler, it might be able to do better, but a user provided vector class wouldn't benefit from the same optimisations.

Note that compilers are not beyond hardcoding invariants of standard library facilities: malloc, C string functions, etc are treated as primitives by the compiler (for example, the result of malloc cannot alias with any other pointer), but for a complex class like vector, it does not seem appropriate and it is better to use compiler hooks in the implementation that are also available to the user (restrict, builtin functions, alias annotations, etc).