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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114388] New: Behavioral change of typeid on xvalues since
 GCC 9
Date: Tue, 19 Mar 2024 06:27:36 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 9.5.0
X-Bugzilla-Keywords: wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: de34 at live dot cn
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114388

            Bug ID: 114388
           Summary: Behavioral change of typeid on xvalues since GCC 9
           Product: gcc
           Version: 9.5.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

IIUC this program behaves differently in C++98 and C++11 due to WG21 N3055
(https://wg21.link/n3055):
```
#include <typeinfo>
#include <cstdio>

struct B {
  virtual ~B() { std::puts("~B"); }
};

struct D : B {};

struct WrapB {
  B b;
};

struct WrapD {
  D d;
};

int main() {
  typeid(true ? WrapB().b : WrapD().d);
}

```

Before C++11/N3055, the operand of the typeid expression shouldn't be evalu=
ated
because it's a rvalue. But since C++11 it should be evaluated because it's a
glvalue of a polymorphic class type.

It's curious that GCC behaves consistently on this in C++98 and C++11 modes.
Per https://godbolt.org/z/7oGdjdMK7, it seems that GCC 8 (and former versio=
ns)
applied the C++98 rules to C++11 and later modes, and GCC 9 (and later veri=
ons)
applied the C++11 rules to C++98 mode.

For later versions, I'm not sure whether this is a bug (in C++98 mode) or an
intentional backporting.=