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From: "barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/112591] New: variant allows for creating multiple
 empty objects at same address
Date: Fri, 17 Nov 2023 16:02:49 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112591

            Bug ID: 112591
           Summary: variant allows for creating multiple empty objects at
                    same address
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider:

#include <variant>
#include <iostream>

struct Empty { ~Empty() {} };
struct Sub : Empty { std::variant<Empty> e; };

int main() {
    Sub v;

    Empty* base =3D &v;
    Empty* obj =3D &std::get<Empty>(v.e);

    std::cout
        << "base=3D" << (void*)base
        << " obj=3D" << (void*)obj
        << " matches=3D" << (base =3D=3D obj)
        << '\n';
}

In C++17 mode on trunk, base and obj have the same address.
In C++20 mode up through 11.4, they have the same address. Since 12.1, they=
 do
not.

If Empty were trivially destructible, they never have the same address.=