[Bug c++/109039] New: Miscompilation with no_unique_address and bitfields

["coillol at yandex dot ru" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109039

            Bug ID: 109039
           Summary: Miscompilation with no_unique_address and bitfields
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: coillol at yandex dot ru
  Target Milestone: ---

g++ miscompiles the following code starting from version 12.1 (-std=c++20, any
optimization mode).


```
#include <cassert>

struct X {
    short x0 : 7;
    short x1 : 8;

    X() { init(); assert(get() == 3); }

    void init() { x0 = 1; x1 = 2; }

    int get() { return x0 + x1; }
};

struct S {
    [[no_unique_address]] X x;
    char c = 0;
};

int main() {
    S s;
    assert(s.x.get() == 3);
    return 0;
}
```

Program hits assertion in main when executed. Links to godbolt:
https://godbolt.org/z/xG186673P (12.1), https://godbolt.org/z/MaP56rfhe
(trunk).