[Bug c++/28907] New: Wrong pointer cast with multiple inheritance

["simark at simark dot ca" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28907

            Bug ID: 28907
           Summary: Wrong pointer cast with multiple inheritance
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: c++
          Assignee: unassigned at sourceware dot org
          Reporter: simark at simark dot ca
  Target Milestone: ---

$ cat test.cpp                                                       
struct Base1 {
  int b1 = 0x1111;
};

struct Base2 {
  int b2 = 0x2222;
};

struct Derived : public Base1, Base2 {
  int d = 0x3333;
};

int main() {
  Derived d;
  return 0;
}
$ g++ test.cpp -g3 -O0                                               
$ DEBUGINFOD_URLS= ./gdb -nx -q --data-directory=data-directory a.out -ex "b
15" -ex r
Reading symbols from a.out...
Breakpoint 1 at 0x1165: file test.cpp, line 15.
Starting program: /home/simark/build/binutils-gdb/gdb/a.out 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".

Breakpoint 1, main () at test.cpp:15
15        return 0;
(gdb) p &d
$1 = (Derived *) 0x7fffffffdcfc
(gdb) p (Base2 *) &d
$2 = (Base2 *) 0x7fffffffdd00
(gdb) p (Derived *) (Base2 *) &d
$3 = (Derived *) 0x7fffffffdcf8


Pretty sure that $1 and $3 should be equal.

Might be related to https://sourceware.org/bugzilla/show_bug.cgi?id=20285.  Not
quite the same, but it might be the same root cause.

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