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

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

[danakj at orodu dot net]

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

            Bug ID: 110245
           Summary: constant evaluation fails with uninitialized union and
                    default constructor
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danakj at orodu dot net
  Target Milestone: ---

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

struct C {
    constexpr C() = default;

    Store s;
};

int main() {
    constexpr auto c = C(); //
}
```

This should compile but does not, GCC complains that it refers to an
incompletely initialized variable. Union members are not required to be active.

https://godbolt.org/z/G9vcosTG1

Reproduced on GCC trunk and GCC 11.

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

[danakj at orodu dot net]

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

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

[pinskia at gcc dot gnu.org]

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So if I read: https://cplusplus.github.io/CWG/issues/2558.html there is still
an open question about "Union types shall be initialized such that they have an
active member in the result of a constant expression" but I am not 100% sure
here.