[Bug c++/105407] New: std::construct_at during constant evaluation does not zero-initialize

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105407

            Bug ID: 105407
           Summary: std::construct_at during constant evaluation does not
                    zero-initialize
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This example program:
```
#include <memory>
#include <iostream>

struct A {
    int x = x;
};

constexpr int foo() {
    A a{1};
    std::construct_at<A>(&a);
    return a.x;
}

int main() {
    constexpr int v = foo();
    std::cout << v << foo();
}
```
shall print "00" (as Clang and MSVC do), because before "x=x" std::construct_at
must zero-initialize (a), see https://eel.is/c++draft/dcl.init#general-9.1.2

GCC prints "10" (demo: https://gcc.godbolt.org/z/eoEr5KMjM) meaning that it
behaves correctly in run-time, but not during constant evaluation.