In the following program, in a constant expression, a temporary object of A is created with all fields initialized, and then function f creates another object of A at the same address, skipping (re)initialization of the field x, which is read afterwards:
```
#include <memory>

struct A {
    int x;
    constexpr A() {}
    constexpr A(int xx) : x(xx) {}
};

constexpr int f(A && a) { 
    a.~A();
    std::construct_at<A>(&a);
    return a.x; 
}

static_assert( f(A{5}) == 5 );
```
This code is accepted by GCC, but other compilers complain about uninitialized object read. Demo: https://gcc.godbolt.org/z/sPGjj75Md

Related discussion: https://stackoverflow.com/q/70820127/7325599