const member and assignment operator. How to avoid the undefined behavior?

Question

I answered the question about std::vector of objects and const-correctness, and received a comment about undefined behavior. I do not agree and therefore I have a question.<

Answer 1

First: When you make a data member const, you're telling the compiler and all the world that this data member never changes. Of course then you cannot assign to it and you certainly must not trick the compiler into accepting code that does so, no matter how clever the trick.
You can either have a const data member or an assignment operator assigning to all data members. You can't have both.

As for your "solution" to the problem:
I suppose that calling the destructor on an object within a member function invoked for that objects would invoke UB right away. Invoking a constructor on uninitialized raw data to create an object from within a member function that's been invoked for an object that resided where now the constructor is invoked on raw data... also very much sounds like UB to me. (Hell, just spelling this out makes my toenails curl.) And, no, I don't have chapter and verse of the standard for that. I hate reading the standard. I think I can't stand its meter.

However, technicalities aside, I admit that you might get away with your "solution" on just about every platform as long as the code stays as simple as in your example. Still, this doesn't make it a good solution. In fact, I'd argue it's not even an acceptable solution, because IME code never stays as simple as that. Over the years it will get extended, changed, mutated, and twisted and then it will silently fail and require a mind-numbing 36hrs shift of debugging in order to find the problem. I don't know about you, but whenever I find a piece of code like this responsible for 36hrs of debugging fun I want to strangle the miserable dumb-wit who did this to me.

Herb Sutter, in his GotW #23, dissects this idea piece by piece and finally concludes that it "is full of pitfalls, it's often wrong, and it makes life a living hell for the authors of derived classes... never use the trick of implementing copy assignment in terms of copy construction by using an explicit destructor followed by placement new, even though this trick crops up every three months on the newsgroups" (emphasize mine).