[Bug c++/110828] New: union constexpr dtor not constexpr when used in member array

[Bug c++/110828] New: union constexpr dtor not constexpr when used in member array

[gccbugbjorn at fahller dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110828

            Bug ID: 110828
           Summary: union constexpr dtor not constexpr when used in member
                    array
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugbjorn at fahller dot se
  Target Milestone: ---

The following code does not compile with g++ 13.1 or g++ trunk
82c2a34b2f2c1a06eff672eba2e447b53f35d7b0

union type {
    constexpr type(){}
    constexpr ~type() {}
    int t;
};

struct S
{
    constexpr S() = default;
    constexpr bool f() const { return true;}
private:
    type v[1];
};


static_assert(S{}.f());

The error message is:
source.cpp:16:20: error: non-constant condition for static assertion
   16 | static_assert(S{}.f());
      |               ~~~~~^~
source.cpp:16:23:   in 'constexpr' expansion of '(&<anonymous>)->S::~S()'
source.cpp:7:8: error: '(((type*)(&<anonymous>.S::v)) != 0)' is not a constant
expression
    7 | struct S
      |        ^

Two simple changes can each make it compile.

1. Comment out the constexpr destructor for 'type'
2. Remove [1] from the declaration of member variable S::v such that it is not
an array.

Link to compiler explorer: https://godbolt.org/z/q3KcKrPh1

[Bug c++/110828] union constexpr dtor not constexpr when used in member array

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110828

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Does it work if you move the static_assert into a function scope? If so then
this is probably a dup of PR85944.

[Bug c++/110828] union constexpr dtor not constexpr when used in member array

[gccbugbjorn at fahller dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110828

--- Comment #2 from Björn Fahller <gccbugbjorn at fahller dot se> ---
If I write it in the same way in a function, it compiles.

consteval auto f()
{
    return S{}.f();
}

constexpr auto b = f();


However, if I break it into a constexpr object S s; and return s.f(), it does
not compile, this time because the construction of 's' fails because it refers
to an unititialized variable, regardless of whether the member is an array or
not.

union type {
    constexpr type(){}
    constexpr ~type() {}
    int t;
};

struct S
{
    constexpr S() {}
    constexpr bool f() const { return true;}
    type v{};
};

consteval auto f()
{
    constexpr S s;
    return s.f();
}

constexpr auto b = f();

https://godbolt.org/z/68zY6ecxs

[Bug c++/110828] union constexpr dtor not constexpr when used in member array

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110828

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #1)
> Does it work if you move the static_assert into a function scope? If so then
> this is probably a dup of PR85944.

Yes it does work with:
```
void f1() {
static_assert(S{}.f());
}
```
So yes it is a dup.

*** This bug has been marked as a duplicate of bug 85944 ***