[Bug c++/110245] constant evaluation fails with uninitialized union and default constructor

[danakj at orodu dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110245

--- Comment #1 from danakj at orodu dot net ---
Note that GCC has no problem with doing this constexpr construction if the
object is not stored as an lvalue.

```
struct Store {
    constexpr Store() {}
    union {
        int i;
    };
    bool b = false;
};

struct C {
    constexpr C() = default;

    Store s;
};

int main() {
    constexpr auto c = C(); // Fails.
    constexpr auto b = C().s.b; // OK
}
```

https://godbolt.org/z/31MTenKoo