[Bug c++/98423] New: The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[Bug c++/98423] New: The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[xmh970252187 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

            Bug ID: 98423
           Summary: The defaulted default constructor defined as deleted
                    when one of variant member has a default member
                    initializer
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xmh970252187 at gmail dot com
  Target Milestone: ---

struct A{
    A(){}
};
union C{
   A a;
   int b = 0;
};
int main(){
    C c;
} 

GCC reports the default constructor for union `C` is defined as
deleted.  However, the relevant rule in the current c++ standard says
that:
> A defaulted default constructor for class X is defined as deleted if:
>> X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
>> X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer

In simple, as long as one of these variant members has a default
member initializer, then the default constructor for the containing
class will not be defined as deleted.  In this example, the variant
member `b` has a default member initializer, hence `C::C()` should be
defined rather than deleted. Is it a bug?

GCC only agrees this code is supported:
struct A{
    A(){}
};
union C{
   A a{};
   int b;
};
int main(){
    C c;
}

If it is the right behavior, then the relevant rule should be described as:
> X is a union that has a variant member with a non-trivial default constructor and no default member initializer is supplied for the variant member.

However, the rule does not say this.

[Bug c++/98423] The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org,
                   |                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think it isn't solely about the process_subob_fn
      if (TREE_CODE (arg) == FIELD_DECL
          && TREE_CODE (DECL_CONTEXT (arg)) == UNION_TYPE)
        {
          if (deleted_p)
            *deleted_p = true;
          if (diag)
            error ("union member %q+D with non-trivial %qD", arg, fn);
        }
diagnostics that probably shouldn't be performed if the union type has nsdmi on
any member, isn't it also the:
  if (constexpr_p && !DECL_DECLARED_CONSTEXPR_P (fn))
    {
      *constexpr_p = false;
      if (diag)
        {
          inform (DECL_SOURCE_LOCATION (fn),
                  SFK_DTOR_P (sfk)
                  ? G_("defaulted destructor calls non-%<constexpr%> %qD")
                  : G_("defaulted constructor calls non-%<constexpr%> %qD"),
                  fn);
          explain_invalid_constexpr_fn (fn);
        }
    }
diagnostics (why would a default constructor of a union be non-constexpr just
because some variant member without nsdmi has non-constexpr default
constructor), perhaps the spec_p stuff too.
So for UNION_TYPE we probably want to search for nsdmis in another loop at the
start of walk_field_subobs and punt some tests for some special members if
there are any nsdmis.

[Bug c++/98423] The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is CWG2084 btw.

[Bug c++/98423] The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-07-20
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

[Bug c++/98423] The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |13.0
         Resolution|---                         |FIXED

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 13, I mistakenly put PR 94823 in the ChangeLog instead of this
one.

[Bug c++/98423] The defaulted default constructor defined as deleted when one of variant member has a default member initializer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98423

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jens.maurer at gmx dot net

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 106150 has been marked as a duplicate of this bug. ***