Violation of const-correctness [placement new] and undefined behavior

Question

I have read the following: Placement new breaks consts and references? and by reading the slides from Matthis Kruse at the C++ Meeting I am confused regarding the following code

struct U { const int i; };
std::vector<U> B;
B.push_back({ 1 });
B.clear();
B.push_back({ 2 });
const int v = B[0].i; // <- Undefined Behavior

B.clear() only destructs the objects, and B.push_back({2}) uses placement new to construct a new object at the start of the memory location B has allocated.

1. I don't understand quite why the access B[0].i is undefined behavior.
2. Could the compiler have cached B[0].i and possibly output 1?
3. What has std::launder to do with all this? Is it a tool to really make sure these kind of compiler optimizations cannot happen?

The standard libcxx [llvm] implementation:

template <class Tp, class Allocator>
inline typename vector<Tp, Allocator>::reference
vector<Tp, Allocator>::operator[](size_type n)
{
   assert(n < size(), "vector[] index out of bounds");
    return this->__begin_[n];
}

is the same as in the slides.

References from std 2017:

• 8.3 (Possible violation)
• 8.4 (mentioning std::launder)