[Bug c++/107039] New: GCC not diagnosing UB in constant expression

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107039

            Bug ID: 107039
           Summary: GCC not diagnosing UB in constant expression
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

GCC doesn't give diagnostic for the following program while clang and msvc do.
Demo: https://godbolt.org/z/o3K57fjeE

```
struct S
{
    int a, b;
};

constexpr void fn( S &s )
{
    (&s.b)[-1] = 123;
}

constexpr auto test = ([]{
    S s;
    fn(s);
}(), 0);
```

There should be a diagnostic for the above code.