[Bug c++/112439] New: Modification of a member overlapping with a [[no_unique_address]] member in the constructor is incorrectly rejected

["de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112439

            Bug ID: 112439
           Summary: Modification of a member overlapping with a
                    [[no_unique_address]] member in the constructor is
                    incorrectly rejected
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

GCC starts (incorrectly) rejecting the following code snipped since GCC13
(https://godbolt.org/z/faqMahz34). It's wrong to emit an error for this because
c is not const in the constructor body.

```
struct Empty {};

class Foo {
public:
    constexpr Foo(int x, Empty y, int z) : a(x), b(y)
    {
        c = z;
    }

private:
    int a{};
    [[no_unique_address]] Empty b{};
    [[no_unique_address]] int c{};
};

constexpr Foo r{1, {}, 3};
```

It seems that the code is correctly accepted if b and c are made not to
overlap.