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From: "akju at mailinator dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/68051] New: copy constructor does not copy default initialized union member
Date: Thu, 22 Oct 2015 15:24:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68051

            Bug ID: 68051
           Summary: copy constructor does not copy default initialized
                    union member
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: akju at mailinator dot com
  Target Milestone: ---

The assertion at the end of the following code fails erroneously:

#include <string>
#include <cassert>

struct test
{
        union {int v = 0;};
        std::string dummy;
};

int main ()
{
        test x;
        x.v = 1;
        test y = x;
        assert (y.v == 1);
}

According to the assembly listing, it seems that the copy construction of y
uses the default member initializer of v instead of x.v. This behaviour does
not occur if the dummy member is removed causing the struct to be trivially
copyable.