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From: "paulhaile3 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111531] New: Bound member function (-Wno-pmf-conversions)
 with multiple inheritance
Date: Fri, 22 Sep 2023 00:30:23 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.0
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X-Bugzilla-Who: paulhaile3 at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111531

            Bug ID: 111531
           Summary: Bound member function (-Wno-pmf-conversions) with
                    multiple inheritance
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: paulhaile3 at gmail dot com
  Target Milestone: ---

I noticed a bug with bound pointer to member functions that I would like to
report. When using multiple inheritance, the this pointer is incorrect and
results in undefined behavior. See the example below, which is modified from
https://github.com/llvm/llvm-project/issues/22495. The last set of addresses
are incorrect - e.g. this resolves to the address of struct B, instead of A,
which means accessing the data member this->x is undefined.

"""
#include <cstdio>

struct A {
int x;
void f() {=20
    printf("A-in-B [A::f]: %p\n", this);
    printf("address of x: %p, value of x: %d\n", &this->x, this->x);
}
};
struct X { char y; };
struct B : X, A {};

typedef void (B::*b_mp)();
typedef void (*b_fptr)(B *);

int main() {

B b;
b.x =3D 100;
b_mp mp =3D &A::f;
printf("B: [main]: %p\n", &b);
printf("address of x: %p, value of x: %d\n", &b.x, b.x);
printf("A-in-B [main]: %p\n", (A *)&b);
(b.*mp)();
b_fptr fp =3D (b_fptr)(&A::f);
fp(&b);
}
"""

Output:
B: [main]: 0x7fff4772e958
address of x: 0x7fff4772e95c, value of x: 100
A-in-B [main]: 0x7fff4772e95c
A-in-B [A::f]: 0x7fff4772e95c
address of x: 0x7fff4772e95c, value of x: 100
A-in-B [A::f]: 0x7fff4772e958
address of x: 0x7fff4772e958, value of x: 0
Compiled with x86-64 gcc 13.2 with flags "-O3 --std=3Dc++20 -Wno-pmf-conver=
sions"=